Advanced and long-term meditation and the autonomic nervous system: A review and synthesis.

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Published in final edited form as: Neurosci Biobehav Rev. 2025 Apr 9;173:106141. doi: 10.1016/j.neubiorev.2025.106141 Advanced and long-term meditation and the autonomic nervous system: A review and synthesis Idil Sezer 1, 2 , Matthew D Sacchet 2, * Author information Article notes Copyright and License information 1 FrontLab, INSERM U1127, Paris Brain Institute, Pitié-Salpêtrière Hospital, Sorbonne University, Paris, FRANCE 2 Meditation Research Program, Department of Psychiatry, Massachusetts General Hospital, Harvard Medical School, Boston, MA, USA * Corresponding author address: Meditation Research Program, Department of Psychiatry, Massachusetts General Hospital, Harvard Medical School, Boston, MA, USA, meditationadministration@mgh.harvard.edu , (M. D. Sacchet). Issue date 2025 Jun. PMC Copyright notice PMCID: PMC12052481  NIHMSID: NIHMS2074162  PMID: 40204160 The publisher's version of this article is available at Neurosci Biobehav Rev Abstract Meditation has become prominent in both clinical and non-clinical applications for its effects on psychological and physical well-being. Long-term meditators, who have dedicated extensive time to their practice, present a unique opportunity to explore the effects of prolonged meditation training on the autonomic nervous system. Research has reported concomitant activation of both sympathetic (aroused) and parasympathetic (relaxed) branches of the autonomic nervous system during some forms of meditation, leading to the term ‘relaxed alertness.’ However, findings are not consistent, with reports of both sympathetic and parasympathetic activation, sympathetic-only, parasympathetic-only, or temporally variable activations, depending on several factors. This review synthesizes these heterogeneous and seemingly inconsistent results in relation to three explanatory factors: (1) specific classification of style or type of meditation; (2) specific definition of the level of expertise of the meditators; and (3) intra-individual variations within a given meditation practice. When these factors are considered, convergent and meaningful patterns emerge, allowing for a shift from the broad notion of 'long-term' meditation to a more precise characterization of 'advanced' meditation, highlighting skills, states, and stages of mastery developed over time. Our synthesis is particularly useful for understanding both long-term and advanced meditation, as it reveals specific heart rate variability patterns, including very low and low-frequency spectral power peaks, along with cardiac and respiratory coupling. Better characterization of the role of the autonomic nervous system in the context of advanced meditation promises to inform improved meditation training, including training assisted by technology, toward more impactful outcomes. Keywords: Autonomic nervous system, long-term meditation, advanced meditation, heart rate, brain-body interactions, parasympathetic nervous system, sympathetic nervous system, cardiac coherence, cardiac resonance, very low frequency spectral power (VLF), low frequency spectral power (LF), high frequency spectral power (HF), cardiac and respiratory coupling 1. Introduction 1.1. Background and overview of meditation and body contemplation Meditation is a contemplative practice that includes a family of techniques that focus on training mental capacities often including attention, self-regulation, and awareness. One of the most common form of meditation is mindfulness, and it has been defined as training attention and awareness of the present moment without judgement ( Kabat-Zinn and Hanh, 2009 ). Meditation has been shown to foster general well-being and the development of capacities including calmness, clarity and concentration ( Walsh and Shapiro, 2006 ). An old form of Buddhism, the Theravāda , includes the practice of vipassanā meditation, which has been translated into English as insight meditation and is the basis of mindfulness practice and thus the Mindfulness Movement in modern times ( Dahl and Davidson, 2019 ; Samuel, 2015 ). In Pali, the liturgical language of Theravāda Buddhism, ‘ vi ’ means ‘in various ways’ and ‘ passanā ’ means ‘seeing’. In this sense, insight meditation offers practitioners a deeper understanding of their own mental processes and the surrounding world ( Sayadaw, 2016 ). Body contemplation is a hallmark of meditation practice in various discourses. In vipassanā practice, kāyagatāsati , which translates to ‘mindfulness of the body’, is an active awareness of the body; its positions and internal states ( Sayadaw, 2016 ). In this context, the body is a central object in the cultivation of mindfulness. In the background of such cultivation stands the absence of positing a mind-body duality and a lack of concern with a particular physical location of the mind ( Anālayo, 2020 ). Vedic practices, along with other Eastern traditions, also stress the importance of mind-body connectedness (Shraman et al., 2018). Yoga, a school of thought within Hinduism, offers a holistic approach integrating mental and bodily states ( Samuel, 2008 ; Singleton, 2010 ). The practice of adopting body postures called āsanas , has become popularized through modern-day yoga, and has been shown to provide scientifically documented health benefits ( Evans et al., 2009 ; Pascoe et al., 2021 ; Ross and Thomas, 2010 ). Siddhāsana refers to a seated yoga position, which is often instructed in meditation practice ( Shadangi, 2021 ). Beyond Vedic traditions, other contemplative systems also emphasize body awareness. For instance, Buddhist practices, particularly within Zen and Tibetan traditions, emphasize the unity of body and mind, often incorporating specific body postures and breath control as key aspects of meditative practice ( Wright, 2009 ). Similarly, Taoist practices, such as qigong, also highlight bodily awareness and the flow of energy through postures and movement ( Kohn, 2006 ). Reflecting the significance of the body in various teachings, extensive practices have been shown to enhance embodiment, as well as body and proprioceptive awareness in meditators ( Cebolla et al., 2016 ; Guthrie et al., 2022 ; Treves et al., 2019 ; Xu et al., 2018 ). Although meditation practice has been linked to effects on the central nervous system (e.g., Sezer et al., 2022 ), the physiological correlates of mind-body interactions and modulated body awareness, which are major pillars of meditation practices, remain understudied. While we highlight here the role of the body in meditation, it is crucial to recognize that practices not explicitly centered on the body, such as certain forms of mindfulness or concentration-based meditations, can also impact bodily functioning. For instance, mindfulness practices that promote non-reactive awareness of mental events have been found to enhance interoception and body awareness ( Pérez-Peña et al., 2022 ). Additionally, meditation strategies that require engagement of higher cognitive capacities, such as loving-kindness meditation and observing-thoughts (a form of open monitoring (OM)) meditation, have been shown to modulate heart rate variability ( Lumma et al., 2015 ). This suggests a broader connection between cognitive processes and autonomic regulation, even when the body is not the primary focus ( Lumma et al., 2015 ). These findings indicate that meditation can induce bodily-related changes, even without an explicit emphasis on body awareness, highlighting the importance of further investigating the body-related consequences of meditation practices. 1.2. Autonomic nervous system and its role in meditation The autonomic nervous system is part of the peripheral nervous system. It is organized in sympathetic and parasympathetic branches, which regulate respectively the ‘fight and flight’ and ‘rest and digest’ responses ( McCorry, 2007 ). The parasympathetic nervous system releases acetylcholine in organs and tissues which promotes digestion, decreases heart rate, and lowers blood pressure. This system helps the body to conserve energy by promoting relaxation and allows for the restoration of bodily tissues. Parasympathetic activation slows respiration ( Paton et al., 1971 ). In contrast, the sympathetic nervous system releases adrenaline and noradrenaline. These hormones activate the body's fight or flight response, increasing heart rate and blood pressure, and directing blood flow to the muscles, lungs, and brain, while decreasing blood flow to the digestive system ( Esler et al., 1985 ). The sympathetic nervous system additionally causes the release of glucose into the bloodstream which fuels the muscles and brain, enhancing their ability to respond effectively to the perceived threat or demand. Activation of this system increases respiration rate, leading to increased oxygen uptake and carbon dioxide elimination, preparing the body for physical activity. The parasympathetic and sympathetic nervous systems work in a coordinated and complementary manner, balancing each other to maintain the body's physiological functions toward responding appropriately to both internal and external stimuli. The balance of both systems is the basis of body homeostasis ( Jänig, 2022 ); it is called the sympathovagal balance. The autonomic nervous system regulates involuntary bodily functions by modulating changes across bodily systems such as respiratory, cardiovascular, and digestive systems. Certain brain regions, in turn, modulate the activity of the autonomic nervous system, leading to the terminology 'central autonomic system' or 'autonomic brain' ( Beissner et al., 2013 ; Benarroch, 1993 ; Saper and Stornetta, 2015 ). A whole-brain activation-likelihood estimation-based meta-analysis ( Beissner et al., 2013 ) identified brain regions implicated in the core of the central autonomic network, comprising of: the left amygdala, right anterior and left posterior insula and midcingulate cortices. While sympathetic-associated regions dominate executive and salience-processing networks, parasympathetic regions dominate in the default mode network. Hence, central neural processing of autonomic function does not simply involve a monolithic network of brain regions, and instead appears to exhibit elements of task and division specificity. The insular cortex and amygdala, respectively, are implicated in salient processes ( Menon and Uddin, 2010 ) and emotional processes ( Sergerie et al., 2008 ). These regions are involved in higher-order autonomic control. Paraventricular and other hypothalamic nuclei contain mixed neuronal populations with parasympathetic and sympathetic preganglionic efferences ( Benarroch, 1993 ). In the medulla, the nucleus of the tractus solitarius and ventrolateral medulla contain a network of respiratory, cardiovagal, and vasomotor neurons, maintaining basic survival functions ( Ross et al., 1985 ). Vascular baroceptors give afferent feedback to effective brain regions ( Suarez-Roca et al., 2021 ), highlighting bidirectional brain-heart interactions. Consequently, engaging in active mental processes such as meditation may exert enduring effects on the activation levels of the parasympathetic or sympathetic nervous systems, suggesting the potential for sustained modulation of autonomic function through intentional cognitive practices. Meditation has been empirically described as ‘relaxing’ and been renowned for its calming effects ( Ivtzan and Lomas, 2016 ; Kushner and Marnocha, 2008 ). These effects suggest parasympathetic effects of meditation on the body. Contradictory results of increased arousal through sympathetic activation have emerged in the literature, coining the term ‘relaxed alertness’ for meditative states ( Britton et al., 2013 ). Relaxed alertness is used to describe the duality of a relaxed, quiet, still body and an active and focused mind. A prior systematic review found that neurophysiological EEG signatures associated with mindfulness practice are increased alpha and theta band power, which were interpreted as neural correlates of relaxed alertness ( Lomas et al., 2015 ). Kerr and colleagues found evidence of heightened alpha top-down modulation during mindfulness-based cognitive practices in clinical contexts ( Kerr et al., 2011 ). Their framework suggests that directing attention to breath and bodily sensations enhances their weight in attentional processes, diminishing attention focused on other ongoing functions. This, within clinical contexts, was theorized to reduce fixation on disordered pain-related attention observed in chronic pain disorders and excessive mental ruminations observed in Major Depressive Disorder ( Kerr et al., 2013 ). Overall, these findings suggest that meditation can modulate autonomic nervous system activity, while contributing to both relaxed and focused mental states. However, it is essential to recognize that meditation includes a diversity of practices, with different meditation techniques leading to different types of outcomes. This underscores the importance of exploring these nuanced patterns to deepen our understanding of how distinct meditative approaches influence autonomic balance. For a comprehensive explanation of meditative techniques reported in the context of this review, please refer to 4. Glossary. For the purpose of clarity, the meditative practices summarized in this review can be grouped into three broad domains: concentration-based practices (such as concentration meditation, samadhi cultivation, transcendental meditation, and dhyana or jhana states), insight-based practices (such as vipassana or Zazen), and practices with an affective component (such as loving-kindness, compassion meditation and heartfulness meditation). Concentration-based practices involve developing attention and mental clarity, often through sustained focus on a single object or thought, while insight-based practices aim to cultivate awareness and understanding of the perceptual experience, typically through mindfulness and observation. Affective-based practices, on the other hand, focus on cultivating particular emotions and emotional states, often loving-kindness, compassion, or empathy. While these categories provide a useful framework, it is important to recognize that certain practices may actively transition between domains and some meditation systems incorporate elements from more than one domain. 1.3. Psychophysiological assessment Scientific observation of physiological signals such as heart rate, respiration and electrodermal activity can provide indication of sympathetic/parasympathetic dynamics. Heart rate, for instance, provides information regarding both parasympathetic and sympathetic activity as the heart is innerved by both systems ( Hildreth et al., 2009 ). Sophisticated analysis methods, including overlapping sliding windows and spectral analysis, allow researchers to quantify physiological signals in time-domain, frequency-domain, and non-linear measures ( Litvack et al., 1995 ; Spiers et al., 1993 ). These techniques provide a comprehensive view of physiological dynamics, offering insights into the complex interplay between sympathetic and parasympathetic activity. Skin is the only organ innerved by exclusively the sympathetic nervous system ( Greaney and Kenney, 2017 ; Vetrugno et al., 2003 ), making electrodermal activity a direct marker of sympathetic arousal. Electrodermal activity, also named galvanic skin response or skin conductance consists of two components: the tonic component (baseline activity) and the phasic component (event-related skin conductance responses, SCR) (Benedek and Kaernbach, 2010). Phasic SCRs can result from skeletal muscle activation (e.g., brainstem reticular formation) or emotional responses (e.g., amygdala and prefrontal cortex), while tonic EDA changes may reflect thermoregulatory sweating (thalamic activation). Phasic SCRs can result from skeletal muscle activation (including activation by the reticular formation of brain stem) or emotional responses (by prefrontal cortex (PFC), amygdala, basal ganglia) while tonic EDA changes may reflect thermoregulatory sweating (thalamic activation) ( Posada-Quintero and Chon, 2020 ; Qasim et al., 2022 ). Respiration has a crucial role in regulating physiological functions through autonomic balance. Studies have reported modulation of breath rate and amplitude as a result of meditation. Changes in these physiological parameters, in this sense, indicate autonomic nervous system modulation, often observed as slowed and deepened breathing patterns during meditation. Reduced breath rate and increased breath amplitude are induced by parasympathetic activation ( Russo et al., 2017 ). Sensors such as respiratory belts or impedance sensors measure these parameters, providing real-time data on breathing dynamics. With respect to heart rate, the temporal distance between each beat, known as the inter-beat interval (IBI) or also known as RR intervals for consecutive R-peaks, can be measured. Heart rate variability (HRV) refers to the variation in the time intervals between successive heart beats. High HRV has been related to psychosocial and physical wellbeing ( Kristal-Boneh et al., 1995 ; Shaffer et al., 2014 ; Togo and Takahashi, 2009 ; Zulfiqar et al., 2010 ). Time domain measures calculate temporal variations between peaks. Frequency domain measures of HRV involve analyzing the signal’s power spectral density after interpolating R peaks. This approach partitions the HRV signal into specific frequency bands: very low frequency (VLF) intervals ranging from 0.01 to 0.04 Hz, low frequency (LF) intervals from 0.04 to 0.15 Hz, and high frequency (HF) intervals from 0.15 to 0.4 Hz.( Zhong et al., 2007 ). The spectral power of VLF HRV is suggested to reflect residual respiratory activity or thermoregulation, while LF spectral power provides insight into both sympathetic and parasympathetic activation. In contrast, HF spectral power is primarily indicative of parasympathetic activation (Berntson et al., 1993). Another commonly assessed heart-related physiological measure is respiratory sinus arrhythmia, which refers to naturally occurring variation in heart rate that is related to breathing. RSA is strongly influenced by the vagus nerve, which is the primary nerve of the parasympathetic nervous system. The vagus nerve regulates many internal organs, including the heart ( Skytioti and Elstad, 2022 ; Yasuma and Hayano, 2004 ). During inspiration, decreased activity of the vagus nerve accelerates heart rate, and during expiration, increased activity of the vagus nerve allows the heart rate to decelerate. The most commonly held theory on why respiratory sinus arrhythmia occurs suggests that this acceleration/deacceleration mechanism improves blood, tissue, and organ oxygenation and reflects the continuous interplay between both branches of the autonomic nervous system. Within the LF range, particularly at a frequency of 0.1 Hz, there is a notable synchronization between the cardiac and respiratory systems when breathing cycles occur at intervals of 10 seconds (0.1 Hz) for both inspiration and expiration. This phenomenon, known as heart coherence , reflects the alignment of heart rate variability with the respiratory cycle. Heart coherence is a concept used in breathing techniques, where individuals consciously practice breathing at a rate that matches this 10-second cycle to achieve a state of physiological harmony ( Sevoz-Couche and Laborde, 2022 ). Regular practice of paced breathing has been associated with psychosocial wellbeing and is thought to support self-regulation and increased autonomic stability ( McCraty and Zayas, 2014 ). Baroreflex sensitivity is an additional physiologically important measurement that quantifies autonomic control of the cardiovascular system. Generally, it is a measure of the autonomic effector response to a given change in arterial pressure ( Wehrwein and Joyner, 2013 ). It is measured by assessing the relationship between spontaneous fluctuations in heart rate and blood pressure, using sequence method or spectral analysis techniques. The sequence method tracks heart rate variations following blood pressure changes. Spectral analysis, on the other hand, evaluates the relationship between blood pressure variability and HRV by examining how oscillations in blood pressure at specific frequencies correspond to similar oscillations in heart rate. The 0.1 Hz frequency (in the LF band, 0.04–0.15 Hz) is particularly important for assessing baroreflex sensitivity, as synchronized oscillations in blood pressure and heart rate within this range are most reflective of baroreflex function ( Cevese et al., 2001 ). Recent findings suggest that cognitive and attentional demands can modulate baroreflex function, with externally directed techniques enhancing baroreceptor sensitivity and promoting parasympathetic relaxation, while internally focused cognitive tasks may suppress sensitivity and facilitate sympathetic activation ( Reyes del Paso et al., 2004 ). This modulation could explain distinct changes associated with different types of meditative practices. These HRV-derived spectral metrics, most importantly VLF, LF, HF band powers, are temporally sensitive and provide dynamic moment-by-moment information. This is particularly important in identifying state changes during meditation practice, potentially related to the deepness of meditative absorption. Additionally, heart rate, HRV and spectral features can be measured in dynamic settings during ambulation and without being obtrusive for research participants. This is particularly useful for ecologically valid meditation-related research contexts, including assessments during retreats or intensive meditation programs, ( Chang et al., 2012 ; Hao et al., 2017 ; Pratap et al., 2020 ; Sas and Chopra, 2015 ). Overall, this section reviews key psychophysiological measures commonly utilized in meditation research, non-exhaustively including heart rate, respiration, electrodermal activity metrics. Among these, heart rate variability (HRV) spectral features have yielded the most valuable insights into the interplay between sympathetic and parasympathetic activity during meditation practices. Understanding these measures in a broader context is essential for interpreting how meditation influences autonomic regulation and overall physiological functioning. 1.4. Long-term and advanced meditation A promising method of assessing physiological changes mediated by meditation is to focus on advanced meditation, which is defined not only by extensive practice experience, measured by the time spent meditating, but also by a higher level of meditative development, characterized by access to deeper phenomenological states and traits. This distinction sets advanced meditators apart from those engaged in long-term meditation practice, which may involve sustained and consistent engagement with meditation techniques over extended periods, typically years rather than months or weeks, without necessarily achieving the same depth of meditative experience ( Sacchet et al., 2024 ). Through time and mastery of meditation practice, advanced meditators achieve altered states of consciousness and ways of being that are associated with distinct cortical profiles ( Chowdhury et al., 2025 , 2023 ; Ganesan et al., 2024 ; Sacchet et al., 2024 ; van Lutterveld et al., 2024 ; Yang et al., 2024 , 2023 ). Advanced meditation promises to provide important new insights into well-being and mental health ( Abellaneda-Pérez et al., 2024 ). As the field of meditation research evolves, it becomes increasingly important to consider the development and phenomenology of meditation to understand the dose-dependent effects of various practices. Notably, the relationship between levels of wellbeing and the duration of practice may follow non-linear dynamics ( Galante et al., 2023 ). This extended commitment implies a deep familiarity with meditation methods and their effects, as well as integration of meditative teachings into one’s lifestyle and perspective. Given these complexities, studying advanced meditation is increasingly crucial, considering the widespread prevalence of altered states of consciousness that are closely linked to such practices ( Wright et al., 2024 ). Understanding the outcomes of advanced meditation requires precise longitudinal designs that take into account various factors, such as initial dosage, the accumulation of ongoing practice, the meditator's developing skills, and complex interactions with the subjects’ past experiences and present environment ( Galante et al., 2023 ). By integrating these insights, researchers can better explore the nuances of advanced meditation and its effects, fostering enhanced well-being and potentially informing therapeutic practices. Notably, prolonged state changes are thought to generate trait changes ( Kiken et al., 2015 ). With respect to neural substrates, both structural changes ( Fox et al., 2014 ; Luders and Kurth, 2019 ) and functional large-scale network changes have been documented in association with long-term meditation practice ( Hasenkamp and Barsalou, 2012 ; Yordanova et al., 2021 ). Conversely, physiological changes, particularly those related to distinct autonomic profiles that may be mediated by meditation practice, remain relatively underexplored. This gap is especially pronounced in research on advanced meditation and physiology, where defining "long-term" or "advanced" practice remains inconsistent, and study dependent. The lack of standardized definitions makes it difficult to compare findings across studies and hinders efforts to establish a cohesive understanding of long-term and advanced meditation’s autonomic effects. Additionally, mechanistic links between meditation and autonomic regulation are seldom addressed in depth, further limiting the ability to identify clear physiological signatures of stages of practice. With this in mind, to identify relevant studies for this review, a comprehensive search was conducted using key terms such as “long-term meditation”, “heart rate”, “electrodermal activity”, “physiology”, “respiration”, “heart rate variability”, and “autonomic nervous system” in PubMed and Google Scholar. No time frame restrictions were applied to the search. A total of 21 studies were identified, and after excluding studies involving meditation-naïve or novice participants (n=5), sixteen studies were included in the final review. In terms of defining “advanced meditation,” no studies explicitly used this term. However, some studies included a small subset of highly experienced practitioners, referred to as 'masters' or similar terms. These participants across studies exhibited distinct physiological patterns and specific phenomenological experiences, and were, for the purpose of this review, related to advanced meditation. The literature surrounding advanced meditation is inherently limited due to the lack of standardized definitions, complicating comparisons between studies. Understanding how meditation impacts autonomic regulation is complex, as it involves multiple parameters that can influence the sympathetic and parasympathetic balance. Prior research has suggested that meditation can induce both physiological relaxation and alertness, depending on several factors, most importantly: (1) the type of meditation practice, (2) the practitioner and (3) the temporal dynamics of the practice. Despite growing interest in advanced meditation and long-term practice, no prior review has comprehensively examined both long-term and advanced meditation through the lens of autonomic nervous system markers. This review aims to address this fragmentation by consolidating findings based on these axes, examining physiological signatures of long-term meditation practice, and integrating both consistent and seemingly contradictory findings into a single model toward an understanding of advanced meditation. This review seeks to provide a conceptual framework that will meaningfully evaluate, and provide a foundation for, the science of the physiology of advanced meditation. 2. The autonomic nervous system and long-term meditation 2.1. Types of meditation practice and their physiological relevance Physiological effects of meditation may depend on the type of meditation practice. In contrast to common assumptions both in science and the mainstream that meditation is always relaxing and associated with low arousal, research findings suggest that this is not necessarily the case. For example, some meditative practices associated with developing compassion and meta-cognitive skills have been shown to relate to mental effort and have been associated with physiological arousal mediated by sympathetic dominance. Conversely, other techniques that emphasize breath awareness and relaxation may lead to reduced physiological arousal and a focus on parasympathetic activation. Contradictory results related to physiological arousal or relaxation were observed in Tantric and Zen meditations ( Steinhubl et al., 2015a ; Wu and Lo, 2008 ). Transcendental meditation includes the repetition of a mantra and returning the focus to the mantra when the mind wanders ( Balaji et al., 2012 ; Trama and Cheema, 2016 ). Phenomenologically, practitioners often describe Transcendental meditation as leading to a state of deep relaxation accompanied by an experience of going beyond ordinary thought and mental contents. This state is often characterized by a sense of inner calm, reduced mental noise, and a feeling of restful alertness, distinct from sleep or ordinary rest ( Shannahoff-Khalsa, 2012 ). In Zen meditation, practitioners typically sit in stillness, observing their thoughts without attachment or engagement. This practice fosters a state of open awareness, where individuals cultivate mindfulness of the present moment and a deeper connection to their internal experiences ( Okumura, 2017 ). Increased HF of HRV was also found in inward-attention Zen meditators (n=10, 3 women, mean duration ± standard deviation (SD) : 6 ± 3.2 years) ( Wu and Lo, 2008 ). This effect is in line with parasympathetic dominance and increased organ oxygenation by the respiratory sinus arrythmia. Studies of both Tantric and Zen traditions have reported sympathetic autonomic activation: increased heart rate and breathing rate ( Corby et al., 1978 ; Hirai, 1974), whereas other studies report parasympathetic autonomic relaxation with decrease in heart and breathing rate ( Elson et al., 1977 ; Kasamatsu and Hirai, 1966 ; Sugi and Akutsu, 1968 ). These seemingly inconsistent results could be related to different types of meditation within the tradition. For instance, Rinzai Zen emphasizes concentration ( dhyana ) whereas Soto Zen emphasizes sitting in open awareness ( Sogen, 2020 ). In one study ( Lehrer et al., 1999 ), compared to Soto Zen practitioners (n=5), Rinzai Zen (n=6) practitioners were found to breathe at a slower rate and exhibited higher amplitude of LF heart rate waves. However, Soto practitioners in this study had, on average, less practice time than Rinzai practitioners, which may have influenced the results. Rinzai practitioners included a nun, monks and one Zen master who was the leader of a temple. These individuals practiced daily for at least four hours per day. In contrast, Soto Zen practitioners comprised of an undergraduate student, graduate students and a physician, with comparatively less practice history. Their practice duration varied, with some practicing a couple of times a week, others engaging in one month of extensive practice per year, and some practicing occasionally. Thus, while the study aimed to explore these differences, the results may be more attributable to variations in practice experience and depth rather than solely to the type of meditation. The active physiological aspect of yogic traditions may indicate sympathetic activation due to their emphasis on physical activity. However, besides specific movements including bending yoga postures ( Sarang and Telles, 2006 ) associated with sympathetic activation, studies of yoga have found shifts towards HF heart rate oscillations ( Telles et al., 2013 ; Muralikrishna et al., 2012). These HF results are suggestive of increased parasympathetic tone. Of note, concurrent increases in both parasympathetic and sympathetic tone can occur at different levels of the autonomic system ( Telles et al., 1995 ; Telles and Desiraju, 1993 ). Investigators have found parasympathetic activation of the heart (i.e., increased HF band activity), with simultaneously occurring increased electrodermal activity indicative of sympathetic activation ( Telles et al., 1995 ). Additionally, alternating activation of subsystems and homeostatic balance of the autonomic nervous system has been observed during different stages of the meditation practice ( Telles et al., 1995 ; see Section 3 . Temporal dynamics of meditation practice ). Heartfulness meditation is characterized by a heightened state of awareness and emotional connection, engaging practitioners in a deep concentration on their emotional processes ( van’t Westeinde and Patel, 2022a ). During heartfulness meditation, individuals direct their attention inward toward the heart, fostering a profound sense of introspection and emotional awareness. This practice encourages feelings of love, compassion, and forgiveness, allowing practitioners to connect with their inner emotions and cultivate inner peace. This technique can be categorized as a high arousal concentrative practice as it involves intense mental focus and heightened physiological activation. Heart rate spectral power analysis was conducted on 26 experts in heartfulness meditation, as defined by their regular practice over many years (mean duration ± standard deviation (SD) : 13.1 ± 7.4 years) ( Léonard et al., 2019 ). During meditative states, the spectral analysis revealed increased normalized LF power alongside decreased normalized HF power, suggesting signs of sympathetic activation. Heartfulness meditation induced a suppression of global vagal modulation and increased sympathetic modulation and baroreflex activation, corroborating sympathetic activation concomitant to parasympathetic system suppression. Similarly, studies focusing on compassion ( Lutz et al., 2009 ) and vipassana ( Delgado-Pastor et al., 2013 ) meditation types have also reported heightened sympathetic arousal. Those results displayed neural correlates. When heart rate measures were combined with fMRI investigation, compassion meditators experiencing body arousal had increased BOLD activation in somatosensory regions and in the right middle insula region, related to emotional processing (n=10 long-term Buddhist practitioners classified as experts in their study) ( Lutz et al., 2009 ). Vipassana mediated activation was associated with increased attention, measured with greater P3b event-related amplitude following an attentional task (n=10 male experienced vipassana meditators with a minimum of 2 years of regular practice, an average experience of 7.5 years, practicing an average of 15 hours weekly) ( Delgado-Pastor et al., 2013 ). Researchers wanted to actively test the ‘arousal vs relaxation’ hypothesis of meditative practices by comparing experts following two different practices: Vajrayana and Theravada ( Amihai and Kozhevnikov, 2014 ). For the purpose of this comparison, they used the focused attention (FA) vs open monitoring (OM) classification system, commonly employed in meditation research ( Lutz et al., 2015 ). FA practices, often used by beginners, involve concentrating on a specific object (such as the breath, visual points, or bodily sensations) to develop focus and mental stability. In contrast, OM practices encourage a broad, non-reactive awareness, allowing thoughts, emotions, and sensations to arise and pass without attachment or judgment. It is important to note that during a meditative session, practitioners can dynamically alternate between FA and OM modes, adapting their approach to the present moment. Theravada vipassana (sometimes translated as insight , a form of OM) involves maintaining open, non-judgmental awareness of mental and physical phenomena as they arise and pass from consciousness, with an emphasis on their impermanence and interdependence. Theravada samatha (FA) focuses on sustained attention on a single object, such as the breath, to stabilize the mind in a state of reduced distraction ( Farias et al., 2021 ). Vajrayana Rig-pa (characterized as OM by Amihai and Kozhevnikov) involves recognizing and maintaining awareness of the mind's natural, clear, and spacious state, free from conceptual thought and dualistic distinctions. Vajrayana deity visualization (characterized as FA by investigators) focuses on concentrated visualization of a deity or symbolic form to cultivate mental clarity ( Upadhyaya, 1988 ). Investigators aimed to test this comparison because, according to them, Theravada Buddhist teachings emphasize "internally steadying" or stabilizing the "unstable mind" to cultivate quiescence and tranquility ( Walshe, 1995 ). However, insight or vipassana practices, contrary to samatha (calm-abiding), which are also central to Theravada, focus on cultivating discernment into the nature of perceptual experience, highlighting the aspects of impermanence, suffering, and non-self. In this sense, vipassana practice could suggest physiological substrates of arousal and heightened activity. According to Amihai and Kozhevnikov, the Vajrayana meditation practices of deity visualization and rig-pa techniques involve mental engagement strategies that extend beyond achieving a state of calmness, aiming instead to cultivate heightened awareness characterized by non-dualistic thinking ( Upadhyaya, 1988 ). Therefore, according to them, Vajrayana practices are suggested to mediate physiological arousal while Theravada practices would suggest physiological relaxation. Although the aim of the investigators is commendable, we advise caution as entire meditation traditions cannot be generalized as ‘relaxing’ or ‘active’ to test the FA/OM framework ( Sparby and Sacchet, 2022 ). In this sense, long-term practitioners (> 7 years) were asked to meditate following their tradition, with a focused attention (FA) session: samatha meditation for Theravada practitioners and deity meditation for Vajrayana practitioners. samatha and deity meditation were compared to OM: vipassana for Theravada practitioners and rig-pa meditation for Vajrayana practitioners. ECG and EEG recordings were directly acquired in the different meditation centers. Regarding the Theravada meditation, results included increased HF band power, LF/HF ratio decrease, decrease in alpha and beta rhythms during vipassana (Theravada, OM) meditation. During samatha (Theravada, FA) meditation, there was no change in HF band, a decrease in LF/HF ratio and a decrease in alpha band power as well. According to the authors, results across both FA and OM forms of Theravada are similar and align with parasympathetic nervous system mediated relaxation and cortical relaxation. However, the above-described EEG markers, notably the decrease in the alpha band can also be related to cortical arousal, with increased attentional processing ( Kerr et al., 2013 ). Regarding Vajrayana, results include a decrease in HF and beta band power in both deity (FA) and rig-pa (OM) forms. They state that these results are in line with a brain-body sympathetic arousal. Amihai and Kozhevnikov argue that an arousal and relaxation state-based classification system should be used to describe and categorize meditative strategies instead of a system based on attentional focus ( Amihai and Kozhevnikov, 2014 ). Overall, current findings suggest that different styles of meditation practice could be associated with different levels of autonomic modulation. For instance, in a study that actively compared ‘arousal’ versus ‘relaxation’ responses, the authors suggested that participants following Theravada practices, vipassana or samatha, had peripheral markers of parasympathetic activation (relaxation) while observed meditators expert in Vajrayana practices, deity or rig-pa, displayed substrates of sympathetic activation (alertness) ( Amihai and Kozhevnikov, 2014 ). However, the relationship between these practices and physiological responses is far more nuanced, cortical markers of Theravada practice suggesting attentional arousal ( Amihai and Kozhevnikov, 2014 ). Several factors, including the meditator's level of expertise, play a significant role. Furthermore, within each tradition, distinct branches focus on different mental trainings, which may elicit diverse physiological responses. This complexity helps explain findings from another study where vipassana meditation was associated with sympathetic arousal and heightened cortical attentional responses ( Delgado-Pastor et al., 2013 ), seemingly contrasting with the findings of relaxation in the context of vipassana meditation in a particular study ( Amihai and Kozhevnikov, 2014 ). Types of meditation in line with enhanced affective and pro-social processing, heartfulness ( Léonard et al., 2019 ), and compassionbased practices ( Lutz et al., 2009 ) may in some instances evoke physiological arousal, as evidenced by heart rate correlates of sympathetic activation in heartfulness and compassion meditation and increased insula-heart rate coupling during compassion meditation ( Lutz et al., 2009 ). One study examining Yogic practices in the context of dynamic yoga postures found evidence of simultaneous physiological relaxation and arousal, reflecting a balanced engagement of both the sympathetic and parasympathetic systems ( Telles et al., 1995 ). Within a specific tradition such as Zen meditation, the specific type of practice yielded different physiological results, with Rinzai practitioners having increased parasympathetic heart rate spectral correlates compared to Soto practitioners ( Lehrer et al., 1999 ), although differences could be attributed to expertise level. 2.2. Inter-individual variations in physiological responses to meditation Research into physiological differences among practitioners has revealed diverse responses across autonomic and neural correlates, even when practicing the same meditative tradition. These variations highlight how meditators engage differently with their practice, resulting in distinct physiological profiles. Below, we explore several studies that investigated these practitioner-induced differences in meditation, emphasizing how different levels of mastery and attentional focus shape physiological responses. One study ( Peressutti et al., 2010 ) investigated heart rate dynamics specifically during Zen meditation. 19 Soto Zen practitioners (7 Women) had meditation experience ranging from 2 months to 20 years. They participated in breath focused-attention (FA) and open-monitoring (OM) meditation sessions for 20 minutes each. Three different groups of meditators were identified by principal component analysis (PCA) based on the spectral components of their heart rate and respiratory rhythms. Novice practitioners with less than 1 year of practice presented a peak in the VLF or LF range and were not included in the PCA, resulting in 4 groups of Zen meditators. According to them, each group of meditators had different frequency domain peaks and patterns, with higher cardio-respiratory coupling in experts compared to novices. Less experienced practitioners displayed higher and more stable respiratory rhythms whereas experts presented more irregular respiratory oscillations, possibly due to different attentional strategies. This supports Lutz’ framework, which postulates that as meditation experience grows, the need for voluntary attention efforts to attain concentration decreases, and the practitioner gradually reduces the focus to a unique object ( Lutz et al., 2008 ). Overall, based on subjects’ group-level clustering, researchers highlighted 4 levels of Soto Zen meditation practice. Volodina and colleagues ( Volodina et al., 2021 ) used a similar scientific paradigm to investigate group-level differences and variations in peripheral nervous system (with photoplesmography (PPG, measuring heartbeats), electrodermal activity, respiration) and central neural (EEG) activity during Taoist meditation. Meditators (n=13, 5 women, with regular practice for more than 2 years) and meditation-naïve control participants (n=15, 9 women) followed a guided 16-step Taoist meditation recording. Meditators followed diverse practices: yoga, qigong, mantra chanting, breath focusing, stopping the internal dialogue, and Buddhist meditation. Most meditators reported regular practice of more than one type of meditation and all except one were unfamiliar with Taoist meditation. During meditation, the study found no differences in EEG correlates between meditators and meditation naïve controls. However, meditators exhibited increased peripheral relaxation, indicated by increased HRV markers, evidenced by an increased coefficient of variation (assessing HRV) and a decreased stress index. Additionally, meditators displayed changes in respiration with a decreased respiratory rate coupled to an increase in respiratory amplitude. These autonomic markers followed similar trends in the control group but to a lesser extent. The researchers calculated a coefficient of relaxation , combining heart rate variability and prefrontal EEG alpha/theta ratio, to reflect both autonomic and central nervous system relaxation. Calculated coefficient of relaxation indices were used for statistical K-means clustering. The clustering revealed two distinct meditator groups: 'concentrated' and 'relaxed.' The 'concentrated' group showed HRV and stress index metrics similar to controls, with little change during meditation, while the 'relaxed' group displayed greater HRV, lower stress, and a decreased respiratory rate. EEG differences included increased delta power and lower alpha/theta ratios in the 'concentrated' group, and decreased left frontal delta power with higher parietal and occipital alpha/theta ratios in the 'relaxed' group. Of particular interest, there were no significant differences between the groups in terms of age, gender, meditation experience, or questionnaire scores. Authors claim no evidence was found related to the depth of meditation, and the differences in physiological markers could be attributed to two different meditative strategies, relaxation or arousal, which participants might have chosen unconsciously. However, the depth of meditation was not directly assessed in their study (e.g., through a questionnaire such as the Meditation Depth Questionnaire - MEDEQ). While the authors did not make any conclusions regarding meditation depth, it could be hypothesized that the observed differences might relate to the distinct strategies of meditation employed by the participants, with ‘ concentrated’ meditators perhaps requiring more focused attention and ‘relaxed’ meditators reaching a state of greater absorption, where less voluntary attention is needed. The fact that the 'concentrated' group showed HRV and stress indices similar to those of the novice group also suggests that this group may not have reached a deeper meditative state, further supporting the idea of a depth-related difference between groups. This could align with the hypothesis that advanced practitioners may require less voluntary attention as they reach deeper meditative states, aligning with traditional meditation frameworks ( Lutz et al., 2008 ). In the context of this study, although it wasn’t directly assessed, the expertise could be associated with the meditation depth rather than the years of experience. The ‘relaxed’ subgroup exhibited markers of cortical arousal, evidenced by EEG findings, and peripheral relaxation, indicated by cardiac markers. This aligns with descriptions found in traditional meditative texts, such as the Yoga Sutras of Patanjali, which describes a state of dhyana meditation as "uninterrupted flow of concentration" combined with a calm body ( Iyengar, 2002 ), and in Buddhist teachings, where meditation fosters a state of relaxed alertness , a balance between physical stillness and heightened mental awareness ( Wallace, 1998 ). In our current review dhayana has been categorized as a concentration-based technique. Further supporting these observations of inter-individual variations, Telles and Desiraju investigated autonomic changes during Raja yoga practice ( Telles and Desiraju, 1993 ). They investigated differences in random thinking and targeted meditative thinking sessions as well as baselines before the sessions. In the context of Raja yoga, random thinking refers to a mental state where thoughts are allowed to flow in an unregulated state, without conscious control or direction, akin to mind wandering. In contrast, targeted meditative thinking involves focusing the mind on a specific object, thought, or concept, a key element in Raja yoga meditation, which aims to harness and direct mental energy toward a meditative goal. 18 men meditators had between 5 and 25 years of experience in that specific meditation (mean + S.D. = 10.1 + 6.2 years). Notably, autonomic shift was highly subject-dependent: compared to an increase in heart rate for all subjects, changes in respiratory rate, finger plethysmogram amplitude (characterizing to blood flow) and electrodermal activity were fewer and often in opposite directions for subjects practicing the same meditation. However, they were consistent during repeated sessions of a same subject. These individual differences did not seem to be correlated with differences in age, duration of meditation experience, or commitment to meditation. Lehrer and colleagues investigated cardiac spectrum in 11 Zen practitioners, (6 Rinzai and 5 Soto practitioners), noting that the Rinzai practitioners were, on average, more experienced than the Soto practitioners in their sample ( Lehrer et al., 1999 ). Participants underwent 20 minutes of tanden breathing, preceded and followed by 5-minute periods of quiet sitting. Tanden breathing involves slow breathing into the lower abdomen. The authors analyzed the participants’ cardiac spectrum, focusing on the presence of frequency peaks. A 'frequency peak' represents a dominant rhythm in the heart rate variability signal that occurs at a specific frequency, reflecting the body's physiological processes. The majority of subjects exhibited respiration rates within the 0.05 to 0.15 Hz range, corresponding to the cardiac LF band during tanden breathing. Several participants displayed cardiac peaks at 0.1 Hz, which is the resonant peak . This peak reflects the synchronization of cardiac and respiratory systems, both primarly functioning at 0.1 Hz in this case. This cardiac coherence mode, described as a narrowed band with high amplitude peak at around 0.1 Hz, appeared in a subset of practitioners. Physiologically, this could mean a resetting of baroreflex sensitivity. In their study, more experienced Rinzai Zen practitioners had frequent heart rhythm irregularities during and after inhalation ( Lehrer et al., 1999 ). Authors further described the cardiac and respiratory rhythms of a Rinzai Zen master, leader of a temple. The Rinzai master breathed approximately once per minute and displayed a very high amplitude peak in the VLF domain. Phenomenological feeling of warmth during higher states of absorption has been reported by most long-term meditators in their study. The feeling of warmth can appear at the center of the torso or in distal extremities, depending on the practitioner and type of meditation. The role of the VLF band in the cardiac spectrum remains poorly understood in the literature, though its activation has been suggested to be associated with thermoregulation ( Fleisher et al., 1996 ). One study followed novice Qigong meditators (n = 8, 5 women, with 1 to 3 months of practice) and Kundalini meditators (n = 4, 2 women, described by the authors as ‘advanced meditators,’ though the total duration of their practice was not specified) ( Peng et al., 1999 ). Qigong, a traditional Chinese mind-body practice, focuses on cultivating life energy (qi) through slow, deliberate movements, controlled breathing, and meditation. Kundalini meditation, rooted in yogic traditions, focuses on activating and directing energy within the body, particularly from the lower spine, through specific breathing techniques (pranayama), chanting, and meditation. Both practices aim to enhance mental clarity, emotional balance, and physical vitality, though they draw from different cultural and spiritual frameworks. In contrast to previous 0.1 Hz peak findings, researchers observed a slightly lower frequency peak at 0.05 Hz ( Peng et al., 1999 ). These variations in frequency peaks still fall within the LF band (0.04 to 0.15 Hz), this frequency being associated with deeper levels of autonomic modulation. The same team later investigated cardiac and respiratory modulations of long-term Kundalini meditators (n=10, 4 women, mean age 42 years; practice experience ranged from 3 to 15 years and participants practiced up to 5 or more times/week) ( Peng et al., 2004 ). They examined three meditative strategies: relaxation response , which involves a focus on a mantra with relaxed breathing (concentration meditation); breath of fire , a rapid breath technique from the diaphragm (breath control); and bilateral segmented breathing, involving inhalation and exhalation in equal steps (breath control). Across both the relaxation response and segmented breathing, participants exhibited prominent LF 0.1 peaks), with strong cardiac and respiratory coherence and marked respiratory sinus arrhythmia. These findings align with previously described findings and suggest a synchronized autonomic modulation around the 0.1 Hz frequency peak. Variations in peak cardiac frequency among individuals appears to be a consistently recurring effect. Phongsuphap et al. investigated cardiac markers during samadhi state cultivation, ( Phongsuphap et al., 2008 ). Samadhi refers to a deep state of meditative absorption, where the practitioner's mind becomes fully unified and focused. This state is marked by stillness, clarity, and enhanced cognitive and emotional regulation ( Shankman, 2008 ). They recruited 35 meditators. According to the authors, all the meditators were proficient in achieving the samadhi state, with most of them considered to be 'masters.' However, the study does not provide specific details about their meditation experience duration or their training background. They analyzed the cardiac power spectrum of all meditators and discovered that specifically samadhi state was associated with the presence of distinct frequency peaks. In their study, corroborating results from Lehrer and colleagues’ study ( Lehrer et al., 1999 ), most meditators displayed a resonant peak in the LF band. Although most exhibited a LF resonant peak, some samadhi state cultivation meditators had a resonant peak in the HF band. This suggests a parasympathetic activation in that subpopulation of practitioners. Finally, exactly as Lehrer’s study ( Lehrer et al., 1999 ), only one particular meditator had a resonant peak in the VLF, accompanied with slowed heart rate, and a reported subjective experience of warmth. While the previous study on Zen meditators identified individuals exhibiting the VLF peak as masters, this particular study on samadhi state cultivation did not specify individual factors that could account for this unique variation. According to authors, the consistent practice and attainment of the samadhi state strongly implies a high level of meditative expertise. While experiencing or cultivating the samadhi state is sometimes considered an advanced stage in meditation, it’s important to recognize that the depth of this experience and the ease with which it can be accessed can vary. Thus, the classification of a meditator as an ‘advanced’ requires more objective criteria and nuanced understanding of their skill development across different meditative traditions. Articulating what is and is not advanced meditation is an active area of research. One study explored immediate neurological and cardiovascular responses to meditation using wireless sensor technology ( Steinhubl et al., 2015b ). It monitored EEG, blood pressure, heart rate, and HRV in 20 long-term meditators (16 women, self-identified as having a regular meditation practice for over 3 months of= 70 min per week) during a wellness retreat. The classification of this sample as longterm meditators may be open to interpretation, as the inclusion criterion of over 3 months of practice of 10 minutes per day, though consistent, suggests that this group may be less trained compared to other groups reported in the literature. Participants had meditation session focused on mantra recitation. Results showed that meditation produced noticeable EEG changes, notably increased gamma activity, high resting alpha power and a small but significant reduction in mean arterial blood pressure. However, the effects on HRV were inconsistent, with no clear link between EEG changes, HRV, and blood pressure. Authors clearly state that physiological markers of meditation showed great inter-individual variability. Corroborating above described studies ( Lehrer et al., 1999 ; Peng et al., 1999 ; Phongsuphap et al., 2008 ), frequency peaks of heart rate spectrum ranged from HF to LF peaks, depending on the practitioner. It is plausible that the observed HF peaks may correspond to the initial phases of meditation practice, particularly in concentration-based meditation, where practitioners are gradually transitioning into deeper states of focus and physiological regulation. This aligns with the hypothesis that different stages of meditation may engage distinct autonomic patterns. Overall, findings related to physiological variations during meditative experience suggest strong practitioner-dependent effects, notably for heart rate fluctuations. There is preliminary evidence that meditators of the same teaching style experience physiological differences that may relate to their meditative expertise. Surprisingly, prior studies have not shown that findings are correlated to the duration or history of practice, suggesting that reaching significant threshold of meditative practice appears to be unrelated to the total duration of practice. The specific LF peak observed at 0.1 Hz is consistent across multiple studies involving different meditation practices. Lehrer et al.'s investigation into Zazen meditation ( Lehrer et al., 1999 ), as well as Steinhubl et al.'s study on concentration meditation ( Steinhubl et al., 2015a ), both reported a resonant peak at 0.1 Hz, which aligns with findings from Phongsuphap et al.'s research on samadhi state cultivation ( Phongsuphap et al., 2008 ). This 0.1 Hz peak is recognized as a marker of cardio-respiratory coherence, indicating synchronization between the heart and respiratory systems. In two studies, one focused on concentration meditation and on study focusing on Zen meditation (insight) ( Lehrer et al., 1999 ; Phongsuphap et al., 2008 ), one particular meditator had a resonant peak in the VLF with reported subjective experience of warmth. As one of the participants experiencing this phenomenon was described as a Zen master ( Lehrer et al., 1999 ), this particular feature could be an autonomic signature of mastery. 2.3. Temporal dynamics of autonomic and neural activity during long-term and advanced meditation practice 2.3.1. Within-subject temporal differences in physiological metrics during a meditative session Peripheral heart rate metrics, as well as central EEG metrics, provide rich temporal information with increased granularity over other methods (e.g., fMRI). Additionally, heart rate, electrodermal activity, EEG sensors are easy to use in ambulatory contexts (e.g. during retreats). Such measurements can improve assessment of temporal dynamics present during specific meditation practices. Of note, research describes a U-shaped dynamic in physiological markers, explained by gradual entering into a meditative state and gradual exiting and return to baseline ( DeLosAngeles et al., 2016 ). Volodina and colleagues ( Volodina et al., 2021 ) studied a cohort of participants while they listened to a guided Taoist meditation recording. This approach allowed for assessment of temporal variations based on the meditative phase. In this context, the meditative process was separated into a 1) pre-meditation baseline, 2) induction: relaxation, body scan, taking position, 3) attention control focusing on stopping internal dialogue, 4) deep meditative visualization, 5) exist and return to baseline and lastly 6) post-meditation rest. They normalized meditators (n=13, 5 women) and active controls (n=15, 9 women) signals based on each individual’s pre-meditation two-minute baseline. This allowed within-subject analyses comparing baseline to meditative state. To evaluate temporal modulations across the 6 predefined periods, they performed an Adaptive Neyman test (AN-test), a statistical method specifically designed to detect differences across multiple time points and capture underlying temporal dynamics. They observed the hypothesized U-shape dynamic in experts as well as controls, with no group effect. For all participants left and right parietal alpha and beta decreased in 3) attention control and 4) deep meditative visualization phases. The temporal signature of these EEG metrics aligns with previous findings ( DeLosAngeles et al., 2016 ), however, the contrasting changes in alpha activity—where it increases in one study and decreases in another—may be attributed to differences in the meditative practice used (e.g., Taoist visualizations vs. concentration meditation). The gradual decrease followed by increase in was more noticeable in the beta band than alpha band activity. In regard to peripheral autonomic markers, HRV gradually increased during the experiment, particularly in the ‘relaxed’ subgroup of meditators that were able to achieve higher levels of absorption. Additionally, respiration rate gradually decreased, with gradual decline in ‘relaxed’ meditators. ‘Concentrated’ meditators, supposed to be less long-term meditators according to authors, reached the decreased respiration level only towards the end of the meditation session. This could suggest that less long-term meditators or meditators using FA techniques to attain absorption may require more time to achieve similar physiological thresholds in the same session. Compared to EEG correlates, physiological correlates seem to follow gradual incline or decline linear trends rather than U-shape trends. Physiological signal changes are several magnitudes slower compared to EEG signals ( Algra et al., 1993 ; Johansen et al., 2013 ; Yoshizaki et al., 2013 ). It could thus be hypothesized that the gradual return to baseline happens at a much slower pace. For instance, HRV variation has been observed during 24-hour recordings ( Algra et al., 1993 ; Johansen et al., 2013 ; Yoshizaki et al., 2013 ), suggesting that, compared to EEG, the temporal graduations could be different in a meditative session. Telles and colleagues describe four different levels during yogic practices ( Telles et al., 2013 ). The first phase is called ‘cancalata’ which corresponds to random thinking ( Telles et al., 2013 ). The second phase is ‘ekagrata’ , during which the attention is directed towards a series of thoughts not related to meditation ( Sunkaria, 2013 ). The last two phases correspond to meditative phases: dharana , focusing with effort, eventually leading to the final stage: dhyana or effortless expansion. Dhyana is characterized by continuous flow of the mind towards the object chosen for meditation. In this study autonomic variables (electrodermal activity, heart rate, respiration) were assessed in 30 men meditators during those four described phases ( Telles et al., 2013 ). All the meditators were residing in a yoga center and had been practicing meditation on the Sanskrit syllable Om for 30 minutes each day, four days a week. They had a minimum of 6 months of experience in meditation on the syllable Om (group average experience–SD=20.95–14.21 months). This study did not include women participants as autonomic metrics are thought to be influenced by hormonal variations during menstrual cycle. Beginning stages of the practice, ekagrata and cancalata did not exhibit conclusive changes in HRV. In the first phase of meditation (related to FA), dharana , there was a slight decrease in heart rate and electrodermal activity, suggesting a slight shift towards parasympathetic activity. During dhyana , participant exhibited decreased electrodermal activity, decreased heart rate, decreased breath rate, and an increased heart rate variability (indicated by increased NN50 and pNN50 metrics), all suggesting a strong parasympathetic shift. Regarding the cardiac spectrum, dhyana was related to decreased LF power and increased HF power. Overall, the changes were all suggestive of reduced sympathetic activity and/or increased parasympathetic activity, during the deeper meditative state (dhyana) . This particular type of yogic practice in this context (entering to dhyana ) was related to gradually increased physiological relaxation (parasympathetic dominance), correlated to gradual meditative absorption. Sarang and Telles ( Sarang and Telles, 2006 ) investigated autonomic variations during yoga practice in meditators residing at a yoga center (n=42, all men). They followed a guided ‘cyclic meditation’ consisting in the succession of different yoga postures. Cyclic meditation, consisted of five phases, each lasting 5 minutes. Phase 1 began with the recitation of a verse from the Mandukya Upanishad , followed by muscle contractions without moving, supine rest, and a standing posture ( tadasana ) to balance weight evenly, called centering. Phase 2 involved lateral bending to the right ( ardhakatichakrasana ), followed by rest in tadasana with instructions on relaxation and awareness, then bending to the left, followed by tadasana . Phase 3 featured forward bending ( padahastasana ) and backward bending ( ardhacakrasana ), each interspersed with rest in tadasana . Phase 4 transitioned to a supine posture with guided relaxation of different parts of the body in sequence. Phase 5 concluded with supine relaxation, a short prayer, and final rest. This cyclic meditation was compared to a same duration (20 minutes) of supine rest ( Shavasana ). Overall, the 5 phases consisted of 3 phases of yoga practice and 2 phases of guided relaxation. LF power and the LF/HF ratio increased in the second phase of cyclic meditation and was reduced after the practice by comparison with the baseline (pre phase). In contrast, HF power was reduced in the second phase and increased after the practice of cyclic meditation, compared to the pre phase. Heart rate showed an increase in the first three phases of cyclic meditation and was reduced in the fifth phase with a further reduction after the practice of cyclic meditation. In the supine rest session, there was no significant change in the LF power, HF power, LF/HF ratio, and heart rate. The second phase consists of the cyclic meditation practice consists of a sideward bending posture ( ardhakatichakrasana ) and a forward bending posture ( padahastasana ). This postural shift could induce sudden sympathetic activation. Overall, results suggest temporal variations of peripheral signals, across different types of meditation practices. Two studies demonstrate that yogic practices lead to dynamic and phasedependent changes in autonomic variables ( Sarang and Telles, 2006 ; Telles et al., 2013 ), with a consistent trend towards increased parasympathetic activity during deeper meditative states. Both static meditative rest and changing physical postures facilitate rapid shifts between sympathetic and parasympathetic activation, contributing to autonomic regulation and balance. Additionally, variations in EEG patterns have been observed over time during meditation, with decreases in beta power associated with meditative depth during Taoist guided meditation, corroborating previous finds on EEG correlates ( DeLosAngeles et al., 2016 ; Volodina et al., 2021 ). Temporal decreases in respiration rate and elevated heart rate variability were linked to enhanced meditative depth, especially among more advanced meditators ( Volodina et al., 2021 ). Overall, these findings suggest that both autonomic and neural systems are influenced by the phases of the meditative experience. Analyzing results in temporal sequences could provide deeper insights into the neural and peripheral substrates corresponding to the depth of meditation. 2.3.2. Physiology and altered states of consciousness during meditation practice Several studies investigated altered states of consciousness termed ‘Samadhi’ or ‘transcendental’ states. They refer to different states of consciousness, depending on meditative teachings and duration of meditative experience. Both Samadhi and the transcendental state are states of altered consciousness during advanced meditation. Samadhi refers to a state of deep absorption and concentration ( Shankman, 2008 ). The transcendental state refers to a state of pure awareness or pure consciousness that is said to go beyond the ordinary waking state of consciousness ( Trama and Cheema, 2016 ). In this state, individuals often experience a profound sense of unity, timelessness, and stillness, characterized by the absence of thought and ego ( Wahbeh et al., 2018 ). It is seen as a heightened level of perception where the distinction between self and the external world dissolves, allowing for deep insight and connection to a larger reality. Both states are often sought in meditative practices as they are believed to lead to deeper insight, clarity, and well-being ( Balaji et al., 2012 ). Despite their frequent use in scientific literature, both terms require precise definitions for meaningful understanding. To fully comprehend these states, it is essential to examine their developmental trajectories and integrate hypotheses from various traditions with empirical research. The Thin Model ( Wright et al., 2023 ) offers a framework that translates contemplative theories from Buddhist traditions into scientific terms, bridging subjective experiences and objective measurements. This model facilitates the investigation of complex mental phenomena, such as the jhānas, by converting subjective accounts into testable neuroscientific hypotheses. Therefore, while Samadhi and transcendental states are often pursued for their potential benefits, ongoing research must elucidate their phenomenological aspects to enhance our understanding of their physiological underpinnings and to help delineate similarities and differences between different forms of advanced meditation and altered states of consciousness more generally. A systematic review investigated the effect of transcendent states across meditation and contemplative traditions (Vedic, Buddhist, Christian, Judaic) ( Wahbeh et al., 2018 ). The review highlighted several key findings related to respiration and heart rate during reported transcendent states. Regarding respiration, two papers reported lowered respiration rates descriptively ( Banquet, 1973 ; Travis and Wallace, 1997 ) whereas one paper reported significant decrease of respiration rate during transcending experiences (11.4 bpm ± 0.41) compared to other experiences (12.7 bpm ± 0.43) during meditation ( Badawi et al., 1984 ; Travis, 2001 ). Notably, respiratory suspension was a common phenomenon accompanying transcendental states, with expert meditators experiencing numerous episodes of breath suspension, averaging between 13 and 18 seconds ( Farrow and Hebert, 1982 ). That study found significant positive correlations between respiratory suspension episodes with subjective experiences of pure consciousness —a state often described as an awareness completely free from thoughts, perceptions, or sensory input, where individuals experience a sense of undisturbed, silent self-awareness. Regarding heart rate, one study observed significant reductions during transcendent states, with heart rate dropping from 71.4 bpm at baseline to 67.2 bpm during meditation ( Badawi et al., 1984 ). Although there were significant changes in respiratory sinus arrhythmia, heart rate and HRV showed no notable differences between transcendent and other meditative states ( Travis, 2001 ). Another study found an increase in heart rate deceleration during transcendent states, particularly in episodes of respiratory suspension ( Travis and Wallace, 1997 ). Overall, these results seem to highlight a parasympathetic dominance during transcendent states, with effects more pronounced on respiratory rhythms rather than cardiac rhythms. Wu and colleagues investigated HRV in the context of inward-attention meditation as practiced by Zen practitioners to enter transcendental states. Practitioners focus their attention on a specific part of the body, which leads to the release of distracting thoughts, allowing for a clearer and more focused mental state. Similar to results presented in the previous section, Wu et al. found regular oscillations in the LF 0.1 Hz peak, but a notable increase of HF band power, suggesting parasympathetic dominance during inward-attention meditation. The investigators did not disclose specific transcendental states and analyzed the whole meditation section as a transcendental state, and did not assess temporal variations. Another study found cardiac changes of expert practitioners who had experience with Samadhi ( Phongsuphap et al., 2008 ). The 35 long-term meditators, most of them considered masters being able to regularly experience Samadhi, meditated for 10 minutes after 5 minutes of quiet sitting, during 3 or 4 sessions in the span of 4 weeks. They investigated cardiac patterns during the Samadhi state compared to baseline and meditative-non-Samadhi state, resulting in a meditative experience with concentration but without above-described phenomenological changes. Practitioners achieving the Samadhi state had different shift in the frequency band power, in VLF, LF or HF bands, depending on the meditator. Some long-term meditators had a resonant peak in the HF band, some meditators in the LF band and once meditator had a resonant peak in the VLF peak with reported phenomenological feelings of warmth, suggesting a marker of advanced meditative stage. The elevated inter-individual variability aligns with the findings discussed in the previous section and further underscores the necessity of clearly defining the Samadhi state by correlating autonomic physiological findings with practitioners’ self-reported experiences. Several studies ( Lehrer et al., 1999 ; Peng et al., 1999 ; Phongsuphap et al., 2008 ) reported increased irregular cardiac and respiratory rhythm in expert meditators, increased with meditation duration (as seen below in Peressutti et al., 2010 ). Of particular interest, the authors reported that the 0.1 Hz resonant peak disappeared in the second half of the practice, while the cardiac and respiratory systems maintained their coupling in oscillations. This increased variability may reflect more flexible autonomic regulation, as long-term meditators develop greater control over their physiological states, potentially enhancing their ability to achieve deeper meditative states. Such irregular rhythms might signify advanced adaptability of the cardiovascular and respiratory systems, a marker of the body's response to prolonged meditative practice. High variability in cardiac and respiratory patterns is strongly indicative of good autonomic health. Such variability reflects greater flexibility in the autonomic nervous system, allowing the body to adapt more effectively to changing internal and external conditions. This increased flexibility has been linked to enhanced emotional regulation, as it supports the body’s ability to manage adverse outcomes and maintain emotional balance ( Thayer and Lane, 2000 ). Furthermore, high autonomic variability has been associated with a range of profound and beneficial health outcomes, including improved cardiovascular health, reduced stress levels, and overall well-being ( McCraty and Zayas, 2014 ; Shaffer et al., 2014 ). Several studies have emphasized the absence of correlation between the total duration of meditative experience and the subjective or objective assessments of meditative depth. This suggests that experience alone may not reliably predict the depth of meditation and highlights a considerable limitation of the study of ‘long-term’ meditators. Taking this into account, linking physiological signals to meditation depth scores could serve as a more objective way to validate the legitimacy of these depth assessments. Only one study ( Léonard et al., 2019 ) correlated respiration to meditation depth, scored through the MEDEQ. The study found linear correlations between meditation depth and respiration rate during heartfulness meditation, demonstrating the potential for physiological markers to reflect deeper states of meditation ( Léonard et al., 2019 ).The MEDEQ is a standardized tool designed to measure the perceived depth of meditation. It assesses various subjective experiences associated with meditative absorption, such as a sense of time distortion, physical stillness, and heightened awareness. By incorporating such subjective measures alongside physiological data, future research could provide a more comprehensive understanding of meditation depth. This section examined the nuanced nature of altered states of consciousness, particularly samadhi and transcendental states, highlighting the necessity for precise definitions and a systematic exploration of their phenomenological aspects. It reviewed studies indicating that autonomic changes, particularly variations in heart rate and respiration, are influenced by individual differences among practitioners and may correlate with subjective experiences. Importantly, the findings underscored that while certain autonomic markers, such as frequency peaks, are associated with deeper meditative states, the total duration of meditation practice does not consistently predict meditative depth. Furthermore, the elevated inter-individual variability observed in physiological responses reflects the complexity of achieving samadhi and transcendental states. Future studies should explicitly investigate levels of meditative absorption using validated biological and physiological measures, in addition to self-reported experiences. Overall, it appears that different levels of expertise are associated with distinct types of autonomic modulation: periods of respiratory suspension and cardiac frequency peaks. 3. Discussion 3.1. Trends in the literature: Physiology and meditation The reviewed studies reveal several trends related to the type of meditation practiced. Meditation that focuses on meta-cognitive skills, such as concentration meditation ( Phongsuphap et al., 2008 ) and Deity Vajrayana meditation, involving visualization and mantra repetitions ( Amihai and Kozhevnikov, 2014 ) seem to be related to sympathetic activation. Additionally, vipassana meditation ( Delgado-Pastor et al., 2013 ) and emotional processes, particularly compassion meditation ( Lutz et al., 2009 ), and heartfulness meditation ( Léonard et al., 2019 ) seem to be associated with physiological arousal through sympathetic activation. These practices are correlated with increased attentional processes ( Delgado-Pastor et al., 2013 ) and enhanced affective processing ( Léonard et al., 2019 ; Lutz et al., 2009 ). Conversely, meditation that may be more relaxation-focused in Theravada and Yogic practices were more related to parasympathetic activation ( Amihai and Kozhevnikov, 2014 ; Telles et al., 1995 ). However, teachings in Zen and Tantric traditions show contradictory findings among practitioners ( Steinhubl et al., 2015a ; Wu and Lo, 2008 ) ( Figure 3 ). Figure 3. Types of meditative practices categorized by their effect on autonomic nervous system activation. Open in a new tab For context and descriptions, refer to the Glossary and Table 1 . This figure illustrates the diversity of meditative practices and their reported influences on autonomic nervous system activity. Practices such as affective-based meditation techniques, practices with a physical activity component, insight practices and concentration practices have been associated with sympathetic activation. In parallel, insight practices, and concentration practices have shown effects related to parasympathetic activation. Additionally, certain practices with a physical activity component, insight and concentration practices have been observed to engage both sympathetic and parasympathetic systems simultaneously. This categorization underscores the complexity and variability within and across meditative traditions, highlighting that their effects on autonomic nervous system activation cannot be uniformly classified. Figure created with BioRender. Sezer, I. (2024) https://BioRender.com/k04m314 . Most of reviewed studies did not directly correlate physiological findings with levels of meditation experience. Among those that did, no significant relationship was found between the depth of meditation and the duration of practice ( Telles and Desiraju, 1993 ; Volodina et al., 2021 ), suggesting that the experience level and physiological effects do not have a linear relation. Notably, only one study ( Léonard et al., 2019 ) correlated physiological signals to meditation depth, using the MEDEQ. They found linear correlations between meditation depth and respiration rate during heartfulness meditation ( Léonard et al., 2019 ). Overall, given that no study highlighted a linear relation between years of practice and observed physiological changes, these results stress the importance of using the depth of meditative experience, rather than the duration of practice, as a correlation metric. As noted, scientific paradigms should provide clearer, more consistent instructions for meditation practices and improve the reporting of phenomenological experiences ( Sparby and Sacchet, 2022 ). Variations in practitioners' expertise, particularly in attention control, could have led to contradictory findings in some studies. Recognizing this variability, two studies clustered participants. One study clustered participants based on their cardiac responses using PCA ( Peressutti et al., 2010 ) and the other based on their cardiac and neural (EEG) responses using K-means clustering ( Volodina et al., 2021 ). The first study found specific spectral heart rate pattern among practitioners of Zazen meditation, while the latter discovered that practitioners from different meditation traditions tended to employ either 'relaxed' or 'concentrated' styles, regardless of the specific method or duration of their practice. Groups were, as previously stated, uncorrelated to the type of practice nor its duration. Temporal dynamics play a key role during meditative experience. Several studies investigated temporal variations within different stages of Taoist guidance and Yoga practice ( Sarang and Telles, 2006 ; Telles et al., 2013 ; Volodina et al., 2021 ). Results varied intra-individually, depending on the state. Heart rate and breath rate changed individually with increased HRV and diminished breath rate during stages of deeper absorption during Taoist guidance ( Volodina et al., 2021 ). In yogic practice, compared the first stage of random thinking cancalata, maximum changes were seen in autonomic variables (heart rate, electrodermal activity, breath rate) during the state of concentration meditation, dhyana . During dhyana , participants had a decrease in electrodermal activity, a decrease in heart rate, a decrease in breath rate and an increase in HRV, overall suggesting a parasympathetic activation. Another study conducted by Sarang and Telles noted continuous alternation between sympathetic and parasympathetic activation, based on the adopted postures in the cyclic meditation ( Sarang and Telles, 2006 ). To summarize, practice type, the practitioner, temporal variations and depth of meditative experience are important metrics to consider when observing autonomic correlates of meditation. In future studies, it is important to move beyond the classification of results solely based on the meditative traditions in which the techniques are contextualized ( Figure 3 ). 3.2. Relationships between long-term and advanced meditation, autonomic nervous system regulation and its impact on behavior Overall, we found trends related to autonomic correlates of long-term meditation. Of all reviewed papers (n=16, Table 1 ), seven reported a specific power spectral density signature. Four studies reported a LF resonant peak at 0.1 Hz during samadhi state cultivation ( Phongsuphap et al., 2008 ); concentration meditation ( Steinhubl et al., 2015a ), Zazen practice ( Lehrer et al., 1999 ), Kundalini breath exercise ( Peng et al., 2004 ) ( Figure 1 ). These practices can be described as concentration and insight type practices. These studies, investigating autonomic effects on very distinct forms of meditation, all observed a 0.1 Hz cardiac signature, which is linked to cardiac coherence , a phenomenon described as the synchronization of the cardiac and respiratory systems, also known as cardiac resonance ( Sevoz-Couche and Laborde, 2022 ). Specifically, the increase in 0.1 Hz oscillations reflects this synchronized state. When the inspiration and expiration cycles are approximately 10 seconds long, the respiratory system can be deliberately aligned with cardiac activity, promoting this coherence. This cardiac-respiratory synchronization, in turn, is hypothesized to mediate vagally induced increased heart-rate variability and baroreceptor activity during rest, which are thought to help maintain normal blood pressure ( McCraty and Shaffer, 2015 ). Vagal afferent stimulation by respiratory-induced pressure oscillations activates limbic and interoceptive areas leading, in the long-term, to changes in awareness, cognition, stress management and emotion regulation ( Sevoz-Couche and Laborde, 2022 ). Table 1. Studies of autonomic markers of long term and advanced meditation. Study design, meditation specificity, level of experience, summary of major findings and their relationship with duration of practice or meditative depth of each autonomic nervous system and meditation article. N = number of participants. HF = high frequency; LF = low frequency; HR = heart rate; FMI = Freiburg Mindfulness Inventory; MAAS = Mindful Attention Awareness Scale; MEDEQ =Meditation Depth Questionnaire; EDA = electrodermal activity. Time domain measures of heart rate variability offer insight into physiology: (1) the Root Mean of Successive Differences (RMSSD) provides quantification of parasympathetic activity; and (2) the Standard Deviation of Normal-to-Normal (SDNN) quantifies parasympathetic and sympathetic activation. Article Study design Type of Meditation (n=number of participants) Experience Level (mean ± standard deviation) Findings Relationship between Physiology and Duration of Practice or Meditative Depth Léonard et al., 2019 Meditati on and active control (paced breathing) comparison of HRV and respiration Correlati on with psychometri c data (FMI, MAAS, MEDEQ) Heartfulne ss meditation (n=26) Mean ± SD = 13.1 ± 7.4 years.Each participant practiced 1 hour daily for a minimum of 4 years During meditation: ↗ LF (normalized unit (n.u.) and power) ↗ respiration rate ↘ SDNN Sympathetic activation During paced breathing (active control) ↗ HF (n.u.) ↘ LF (n.u.) Parasympat hetic activation No correlation found with duration of practice Significant linear correlation between depth of meditation (MEDEQ) and breathing rate Lehrer et al, 1999 Investiga ting respiration and HRV during tanden breathing (=meditative practice) compared to rest Zazen Rinzai (n=6) and Zazen Soto (n=5) Mean ± SD not stated. Rinzai group, practiced daily for at least 4 hours (monks, nuns, master). The Soto practitioner s in this sample practiced considerabl y less often (graduate students) During meditation: ↗ LF power ↘ HF power Sympathetic activation Rinzai compared to Soto: ↘ respiration rate compared to Soto ↗ LF - LF 0.1 Hz peak: cardiac and respiratory coupling Advanced Rinzai practitioners/ma sters: VLF peak Heart rate irregularities Breathing once per minute Feeling of warmth Telles and Desiraju, 1993 Meditati on compared (HRV, respiration, EDA, EEG) to active control (random thinking) 3 repeated sessions Brahmaku maris Raja yoga meditation (n=18) Mean ± SD = 10.1 ± 6.2 years. Range = 5-25 years of practice During meditation (group-level): ↗ heart rate → EDA(↘ or ↗, depending on participant) → respiration rate (↘ or ↗, depending on participant) Simultaneo us parasympathetic and sympathetic activation at subject-level No correlation found. Autonomic changes dependent of the participants Sarang and Telles, 2006 Alternati ng postures of meditation (yoga) vs supine rest (shavasana) to investigate HRV differences Cyclic meditation = yoga posture and final meditation (n=42) More than 3 months. Mean ± SD = 15.3 ± 13.3 months During meditation: Second phase: ↗ LF power ↗ LF/HF ratio ↘ HF power (decreased vagal tone) ↗ Breath rate. Sympathetic activation Fifth phase & post-meditation ↘ HR ↘ LF/HF ratio ↗ HF power (increased vagal tone). Shift to parasympathetic dominance Not studied Amihai and Kozhevnik ov, 2014 Focused attention (FA) and open-monitoring (OM) hypothesis testing. Compari son to rest of FA Theravada (Shamatha) and OM Theravada (Vipassana) and FA (Deity) and OM (Rig-pa) Vajrayana meditation. ECG and EEG comparison between meditation and rest. FA and OM Theravada (according to authors): Shamatha (n=12) and Vipassana (n=14) FA and OM Vajrayana: Deity and Rig-pa (n=9, same group) Therava da – FA: Shamatha Mean = 10 years Therava da – OM: Vipassana Mean = 12.3 years Vajraya na – FA: Deity Mean = 7.4 years Vajrayana – OM: Rig-pa Mean = 7.4 years Standard deviation not stated. Theravada – FA: Shamatha ↘ LF/HF ratio → HF n.u. (no difference) ↘ Alpha power, ↘ Beta power, ↗ Alpha coherence compared to Vipassana Parasympat hetic dominance, neural arousal Theravada – OM: Vipassana ↗ HF n.u. ↘ LF/HF ratio ↘ Alpha power, ↘ Beta power, ↗ Gamma power ↘ Alpha coherence than Focused attention Theravada Parasympat hetic relaxation response, neural arousal Vajrayana – FA: Deity ↘ HF n.u. ↗ HR n.u. No significant EEG findings Unclear dominance Vajrayana – OM: Rig-pa ↘ HF n.u. ↗ HR → Alpha power, ↘ Beta power, ↘ Gamma power, ↗ Alpha coherence compared to Vajrayana – Focused attention Sympathetic dominance, neural relaxation Not studied Phongsup hap et al, 2008 Long-term samadhi meditatiors and meditation naïve participants (quiet sitting) Clusterin g based on heart rate resonant peak Samadhi state cultivation (concentration) (n=35) Meditation -naives (n=70), quiet sitting Not stated. Described as long-term meditators having experience in achieving the samadhi state, some of them described as masters. During meditation: Depending on the meditator: - HF peak - LF peak - LF 0.1 Hz peak: cardiac and respiratory coupling Simultaneo us sympathetic and parasympathetic activation During samadhi state: - VLF peak Wu et al, 2008 Resting compared to meditative sessions in long-term meditators. Resting compared to baseline in meditation-naïves. Within group, between group and interaction comparisons HR metrics. Inward attention (Zen meditation) Long-term meditators (n=10) and meditation-naïves (n=10) Mean ± SD = 6.0 ± 3.2 years Inward-Attention Meditation (session effect): ↗ HF power ↘ LF power - presence of HF peaks Parasympat hetic dominance Regular oscillations: cardiac and respiratory coupling Rest in long-term meditators (interaction effect group x session): ↗ LF n.u. ↗ HF n.u. ↗ LF/HF ratio ↗ SDNN Overall increased autonomic tone (simultaneous increased parasympathetic and sympathetic amplitude) No difference between groups at rest. Not studied Peressutti et al, 2010 HRV investigation during Zazen. Comparison of. Long-term and novice meditators. Clustering based on cardiac metrics revealed different groups. Soto Zen meditators (n=19): FA and OM Range: between 2 months and 20 years of experience During Zazen Meditation: Group 1 (novice practitioners, <1 year of practice): ↗ peaks in VLF and LF (n.u. and power) ↗ Respiratory rate (stable) ↘ SDNN (reflecting lower HRV) ↗ LF/HF ratio Sympathetic activation Group 2 (intermediate practitioners): ↗ VLF range power ↘ HF power Sympathetic activation Group 3 (‘advanced’ practitioners): ↗ LF power (HRV mainly in LF range) ↗ Irregular LF oscillations in the second half of meditation Simultaneo us sympathetic and parasympathetic activation Advanced practitioners: - LF power - irregular cardiac and respiratory patterns in the second half of meditation Steinhubl et al, 2015 Meditati on-naïves and long-term practitioners, week-long retreat. Ambulatory ECG, blood pressure, EEG Mantra meditation (concentration meditation) and breathing exercises. Long-term (n=20) and naive (n=20) meditators. Meditat ors daily practice mean ± SD = 32 ± 9.23 minutes. Over 3 months of = 70 minutes per week. Meditation-naïves meditators: 0 minutes or not meditated more than once a week during the prior 3 months During meditation: ↗ blood pressure → heart rate ↘ RMSSD LF 0.05 Hz peak Sympathetic activation ↘ Relative gamma power No significant difference observed between meditators and novice meditators Telles et al, 1995 Meditati ve state vs resting state comparison in meditators. Oxygen compustion, HR, skin resistance (finger plethysmogr am) OM meditation (concentration meditation), (n=7) Long-term meditators with experience ranging between 5 and 20 years During meditation: ↘ Heart rate ↗ Finger plethysmogram amplitude Simultaneo us sympathetic and parasympathetic activation Oxygen consumption values of more long-term meditators of the present study were noticeably higher than those of other meditators Telles et al, 2013 Investigating heart rate, skin resistance and respiration during active controls: random thinking, non-meditatative focus, meditative focusing (dharana), and effortless meditation (dhyana) Meditative focusing (dharana, concentration meditation), and effortless meditation (dhyana, insight in the context of the article) (n=30) Meditat iors mean experience ± SD = 20.95 ± 14.21 months. They had a minimum of 6 months of experience During effortless (insight) meditation: ↗ Skin resistance ↘ Heart rate ↘ Breath rate ↘ LF power ↗ HF power ↗ NN50 and pNN50 in HRV Parasympat hetic dominance Focused attention meditation ↗ skin resistance ↘ respiratory rate Parasympat hetic dominance Not studied Peng et al, 1999 Qigong meditation practitioners compared to Kundalini yoga practitioners. Heart rate parameters Qigong (n=8), Kundalini yoga (n=4), controls (n=11), elite athlete controls (n=9) The Qigong meditators were novices in their practice (1–3 months). The Kundalini Yoga meditators were considered ‘advanced’ practitioners by the authors. Their duration of practice was not specified During meditation, in all practitioners: ↗ LF power LF 0.05 Hz peak Regular oscillations: cardiac and respiratory coupling No difference with controls in heart rate oscillations during non-meditative periods Not studied Peng et al, 2004 Kundalin i yoga practitioner’ heart rate and respiration analyses during relaxation response, breath of fire, and segmented breathing Kundalini yoga (n=10). Three different breathing exercises Subjects had 3 – 15 years experience and practiced up to 5 or more times per week During relaxation response and segmented breathing (compared to baseline): ↗ LF power - 0.1 Hz LF peak Regular oscillations: cardiac and respiratory coupling Breath of fire (compared to baseline): ↗ mean heart rate ↘ decrease of cardiac and respiratory coherence Sympathetic activation Volodina et al, 2021 Multimodal assessment (HR, EDA, respiration, EEG) of Taoist meditation. Guided Taoist meditation Long-term meditators (n=13), meditation-naïve controls (n=15) Long-term had regular (several times a week) meditative practice for 2 years or more. Novices should not have any experience in meditation. Experienced meditators had never practiced Taoist meditation Long-term meditators compared to controls ↗ SDNN (of HRV) respiratory rate ↘ indices of relaxation (coefficient of variation HRV x alpha/theta ratio) Subgroups of meditators based on clustering: Relaxed Meditators: ↗HRV indices respiration rate ↘ stress index ↗ occipital and parietal alpha power ↗ alpha/theta ratio ↘ left frontal delta power Parasympat hetic activation Concentrated meditators ↘ HR ↗ stress index ↗ delta power ↘ alpha power ↘alpha/theta ratio ↘alpha/beta ratio No correlation found between the duration and type of previous meditation experience and physiological markers of meditative development. ‘Relaxed’ meditators may be more advanced on a physiological level Delgado-Pastor et al, 2013 Investigated heart rate and EEG evoked-potential coupling in long-term Vipassana meditators. Comparison of Vipassana meditation (n=10) Minimu m of 2 years of regular practice, an average experience of 7.5 years, and an average weekly practice of 15 hours During meditation (Vipassana) compared to baseline: ↗ LF/HF ratio ↗ LF power - 0.1 Hz LF peak ↗ attentional engagement after meditation ↗ autonomic regulation during meditation ↗ P3b amplitudes (to target tone) Sympathetic activation During meditation (Metta) compared to baseline: ↘ LF power ↘ HF power ↗ LF/HF ratio Not studied Lutz et al, 2009 Cardiac and fMRI BOLD activation investigation. Meditative compared to control sessions, long-term meditators compared to controls Compassion meditation practitioners (n=10) and (n=10) controls Long-term compassion meditation practitioners previously completed mental training for 10,000 to 50,000 hours (length estimated as average of 10 hours of practice per day in retreat) which included a variety of meditation practices including compassion meditation Compassion meditation state compared to neutral state: ↗ HR in compassion state for both groups (experts and naïves) ↗ Stronger HR increase in experts compared to novices ↗ Greater positive coupling of heart rate and BOLD in the dorsal anterior cingulate cortex for both groups ↗ HR/BOLD coupling in the right somatosensory cortex and right inferior parietal lobule (IPL) in experts compared to controls → No effect on respiratory rate Sympathetic activation Group by state interaction ↗ stronger HR/BOLD coupling during compassion compared to rest in experts Not studied Open in a new tab Figure 1. LF resonant peak at 0.1 Hz corresponding to cardio-respiratory coupling. Open in a new tab A, B, C, D This is observed during (A) samadhi state cultivation ( Phongsuphap et al., 2008 ); (B) concentration meditation ( Steinhubl et al., 2015a ); (C) vipassana ( Delgado-Pastor et al., 2013 ) and (D) Zazen practice ( Lehrer et al., 1999 ). E, F Cardiac and respiratory systems’ coupling of (E) a Zen master in LF domain ( Peressutti et al., 2010 ) and of a (F) Kundalini long-term practitioner during segmented breathing exercise ( Peng et al., 2004 ). Of note, the respiratory system is the only primitive, survival based physiological function that can be consciously controlled. In animal models, intermittent hypoxia enhanced cardio-respiratory coupling ( Dick et al., 2014 ). Decreased cardio-respiratory coupling has been associated with chronic disorders and poor prognosis ( Fisher et al., 2022 ). Based on these studies, overall physiological synchronization may be related to a reduction in energy expenditure. In other words, this cardiac and respiratory synchronization could represent a more efficient or optimized use of energy and metabolism, which may also contribute to greater degrees of meditative absorption. Cardiac coherence, when viewed in the context of meditative practice, may play a significant role in explaining its broader physiological benefits. Specifically, improved coherence between heart rate and respiratory rhythms is thought to contribute to the reduction of anxiety and the relief of pain These anxiolytic and analgesic effects have been well-documented in various clinical studies on meditation, highlighting how such physiological synchronization may enhance the calming and healing potential of meditation. Studies focused on affective meditative strategies such as heartfulness meditation ( Léonard et al., 2019 ) and compassion meditation ( Lutz et al., 2009 ) have reported increased cardiac coherence. A study of vipassana meditation related to attention control also reported an increased coherence in cardiac spectral power ( Delgado-Pastor et al., 2013 ). This was indicated by increased LF band power, a spectral band power containing the resonant frequency, although the presence of the resonant peak was not specifically stated. These findings were cortically correlated with an increased EEG P3b event-related potential, corresponding to increased attention during vipassana (insight) meditation ( Delgado-Pastor et al., 2013 ). Additionally, there was increased BOLD activation as measured by functional MRI in the somatosensory and middle insular regions during compassion meditation, which is a region thought to be involved in emotion regulation ( Lutz et al., 2009 ). Results were in line with theories of increased physiological coherence during cognitive attentional and emotional processes. It has been hypothesized that improved autonomic function may be related to higher emotion regulation ( McCraty and Zayas, 2014 ). It is suggested that one’s ability to self-regulate moment-to-moment emotional processes is intimately tied to physiology, and the reciprocal interactions among physiological autonomic, as well as cognitive and emotional neural systems. These systems communicate through rhythmic patterns of activity, with heart-brain communication playing a key role in self-regulation. Positive emotions are thought to enhance coherence in bodily processes, reflected in a more stable heart rhythm. This, in turn, supports higher cognitive functions, emotional stability, and calmness. Over time, this creates a new inner baseline that shapes perception, feelings, and behavior ( McCraty and Zayas, 2014 ). Emerging theories emphasize the significance of bidirectional interactions between the brain and the heart, suggesting their involvement in higher-order cognitive processes. According to these theories, these interactions may encompass the collection and integration of information, interpretation, and control of bodily activities ( Candia-Rivera, 2022 ). Such processes are fundamental to both physiological and cognitive functions, ranging from homeostatic and allostatic control to higher-order cognitive functions such as emotion regulation and attention control ( Candia-Rivera et al., 2022 ; Hestad et al., 2022 ; Kumar et al., 2020 ). The neuroscience of brain-heart connectivity posits that embodied cognition may arise from the dynamic interactions between the brain, the body, and the environment, rather than being entirely localized within the brain ( Thompson and Varela, 2001 ). Higher brain-heart connectivity has been reported in the context of emotion elicitation ( Miller, 2019 ), and stress reduction ( Sezer et al., 2024 ). As preliminary results investigated in this review state that there is a co-activation of central and autonomic systems during vipassana and compassion meditation ( Delgado-Pastor et al., 2013 ; Lutz et al., 2009 ), it would be valuable to further explore brain-heart coupling in the context of advanced meditation using state-of-the-art models. Two studies ( Lehrer et al., 1999 ; Phongsuphap et al., 2008 ) reported an even lower frequency peak in the VLF band. In contrast, one study found VLF peak in novice practitioners with 1 year of practice ( Peressutti et al., 2010 ). Although physiological mechanisms underlying VLF increase remain unclear, it has been correlated to thermoregulatory activity mediated by the sympathetic system ( Karayannis et al., 2012 ). In parallel, meditation masters exhibiting increased VLF band power during practice have reported phenomenological feelings of interior warmth ( Lehrer et al., 1999 ; Phongsuphap et al., 2008 ). This strongly suggests that physiological activity and phenomenological experience are aligned, and that the observed VLF peaks could serve as a meaningful correlate of deeper meditative absorption. Future research should test this hypothesis further. Another study reported irregular cardiac and respiratory spectral activity during the later stages of absorption in Soto Zen practitioners ( Peressutti et al., 2010 ). This irregular pattern was experienced after the first half of the practice, during which researchers observed the resonant peak at 0.1 Hz. Although cardiac and respiratory activities in the second half of the practice were irregular, the systems remained notably coupled. This same pattern of irregular and coupled cardiac and respiratory activity was also observed in breathing exercises during Kundalini concentration meditation ( Peng et al., 2004 ). Shift towards VLF peak or coupled and irregular respiratory and cardiac activity could both serve as autonomic biomarkers of advanced meditation ( Figure 2 ). Further studies are needed to provide more information and better understand physiological thresholds of meditation depth. Figure 2. Mechanistic framework of autonomic system markers and their plausible correlations to level of meditative advancement. Open in a new tab HF spectral power increase corresponds to increased parasympathetic activity, observed in early stages of some forms of meditation, while LF peak amplitude reflects simultaneous sympathetic and parasympathetic systems modulation and cardiac and respiratory coupling , as depicted in an exemplary power spectral figure (plot illustration created using Python). The VLF spectral power peak is linked to increased feelings of warmth and is associated with sympathetic system-driven thermoregulation. Advanced meditators exhibit irregular and coupled patterns of cardiac and respiratory activity during the latter half of a meditative session (plot illustration created using Python, adapted from Peressutti et al., 2010 ). Preliminary findings presented in this literature review suggest that this irregular activity as well as the VLF peak both correlate with advanced meditative experience. Figure created in BioRender: Sezer, I. (2024) https://BioRender.com/a26j858 . 3.3. Meditation research: past, present and future The scientific study of meditation’s physiological and neurophysiological effects has undergone notable methodological and technological shifts over the past several decades. Early research in the 1970s primarily focused on physiological markers of transcendental meditation, identifying modulations in respiration, heart rate and electrodermal activity ( Wallace, 1970 ). For example, EEG studies of transcendental meditation revealed increased alpha and theta wave activity as signatures of meditative states ( Banquet, 1973 ). However, this work remained peripheral to mainstream scientific discourse. It wasn’t until the late 1990s that meditation research gained wider recognition, marking what we call the first wave of modern meditation science ( Sacchet et al., 2024 ). In the first wave, from about 1995 until 2005, meditation research gained mainstream attention, largely driven by clinical applications such as Mindfulness-Based Stress Reduction (MBSR) and other structured mindfulness programs ( Kabat-Zinn, 2003 ). Concurrently, research further built on autonomic nervous system regulation, with heart rate variability HRV emerging as a key metric for assessing meditation’s influence on vagal tone and sympathetic activity ( Lehrer et al., 1999 ; Peng et al., 1999 ). The second wave of meditation research (approximately 2005–2020) focused on identifying the neural and physiological mechanisms mindfulness-related health benefits, with the integration of heart rate measures to neuroimaging techniques, such as functional magnetic resonance imaging (fMRI), as well as heart-rate-EEG integration, has allowed researchers to investigate brain-body interactions in greater detail ( Delgado-Pastor et al., 2013 ; Lutz et al., 2009 ). We are now entering a third wave of meditation research (approximately 2020–present), with a focus extending beyond mindfulness clinical applications to investigate advanced meditation, including their potential for self-transcendence ( Sacchet et al., 2024 ). This new phase includes the use of cutting-edge technologies, including multimodal neuroimaging and machine learning, to refine the assessment of meditation mastery. In parallel, advancements in wearable technology and portable applications are now transforming meditation research by enabling real-time physiological monitoring in naturalistic settings such as intensive retreats. Modern wearable devices can continuously track HRV, respiration rate, electrodermal activity, and EEG, allowing for more ecologically valid studies of long-term and advanced meditators ( Steinhubl et al., 2015a ). These tools not only facilitate large-scale data collection outside laboratory environments but also open the door for biofeedback interventions aimed at optimizing meditation practice ( Jomaa et al., 2024 ). Additionally, multimodal approaches that combine EEG, fMRI, and peripheral physiological markers offer unprecedented precision in capturing the temporal dynamics of advanced meditation. As technology advances, there is growing potential to refine meditation-based interventions by clearly assessing mastery of practice. Rather than relying solely on self-reported expertise or practice duration, future research may leverage physiological biomarkers to develop objective measures of advanced meditation states. Identifying these biomarkers could help individuals progress more efficiently through meditative development through closed-loop technologies. In this review, we identify VLF peaks and irregular cardiac and respiratory coupling patterns as potential biomarkers, which, when correlated with subjective reports and meditation depth scores (such as MEDEQ scores), could provide deeper insights. Neuromodulation through stimulation ( Abellaneda-Pérez et al., 2024 ), or real-time biofeedback and neurofeedback from wearable devices, could help practitioners and clinicians tailor meditation techniques to individuals, potentially enhancing both scientific understanding and clinical applications. Moving forward, the integration of these emerging technologies with traditional meditation research methodologies could likely shape the next edge of contemplative science. 3.4. Limitations The field of physiology research on meditation has several limitations. There is a lack of clear definition of important parameters which has slowed scientific progress. Meditation is a vast domain with several ancient teachings adopting distinct mental strategies. Some scriptures stress the importance of intense mental exercise related to a meditative object whereas, in contrast, others emphasize cultivating mental quietness and calmness. Affective-based teachings have a key emotional component not present in others. In this line, meditation techniques need to be carefully classified. Consequently, there is a pressing need for careful classification of meditation techniques. With these current shortcomings in mind, we developed an activity-based phenomenological classification system ( Sparby and Sacchet, 2022 ). Our integrated model outlines the central aspects of meditation as mental activity. Meditative activities can be combined and unified in the activities of observing, producing, and being aware. All meditative activities are unified in awareness of awareness. We distinguish between the context surrounding the activity and the activity itself. The context encompasses the understanding and beliefs associated with meditation, while the activity refers to the actions performed during meditation. The model suggests that the same activities can be performed within different contexts, such as secular or spiritual, without necessarily changing the actions themselves. Spiritual context involves concepts like transcendence, ego dissolution, and the pursuit of enlightenment, while secular context may focus on relaxation and stress reduction. The model suggests that these contexts can be complementary rather than contradictory, with physiological responses to relaxation being seen as external manifestations of mental relaxation in a spiritual context. This system could be used to categorize meditative approaches and classify experience phenomenology and its physiological signature. One pertinent illustration in the context of this review is the VLF peak associated with experiences of warmth. Building on this foundational understanding, we propose an additional framework for meditative development that emphasizes the need to assess factors such as meditation dosage, techniques, individual characteristics, and cultural contexts. It advocates for developing new measurement tools and conducting longitudinal studies, while also drawing on contemplative traditions and involving diverse practitioners to ensure research relevance and accuracy, all within a framework of transparency through open-access publication ( Galante et al., 2023 ). While meditation is often associated with positive outcomes, intensive practice, particularly in retreat settings, may lead to challenging psychological and physiological experiences. Reports have documented instances of distressing states, including anxiety, depersonalization, and emotional upheavals ( Britton et al., 2021 ). These effects underscore the importance of monitoring individual responses to practice and integrating safeguards, such as professional guidance and mental health support, into meditation-based interventions. Future research should explore both the benefits and potential risks of deep meditative absorption, ensuring a balanced ethical and dose-dependent approach to contemplative science. In case of discrepancies between subjective meditator experiences and objective measurements, such as body and brain states, contemplative theory can be utilized to inform scientific inquiry ( Wright et al., 2023 ). Our proposed Thin Model translates contemplative theories from the wisdom traditions into a functionalist model of the mind, demonstrating its immediate explanatory power. The model facilitates connecting higher-level mental phenomena to lower-level brain states without oversimplification. By exemplifying this approach with specific meditative states known as jhānas, we illustrate its potential for generating testable neuroscientific hypotheses. Knowing the central aspect of body awareness in meditation, the model could be extended to include these newly refined markers of autonomic function to its already-existing definitions. 3.5. Conclusions and perspectives Among the physiological signals used to evaluate autonomic activity—including heart rate, respiration and electrodermal activity—results suggest that heart rate-based approaches are the most dependable and reproducible, especially concerning extracted spectral HRV data. These metrics open a wide range of scientific applications as they can be used in ambulatory ecological settings. Identified specific features, notably frequency peaks, could be used to develop approaches for real-time assessment and monitoring of advanced meditative states. Additionally, these physiological biomarkers could enhance meditation teaching through biofeedback and neurofeedback-based trainings, thereby contributing to the widespread adoption of meditation, particularly in clinical settings. However, researchers need to be wary of weak study designs in meditation research ( Galante et al., 2023 ; Van Dam et al., 2018 ). Prior research suggests a range of experiences with regular meditation practice, from positive to challenging. Further studies need to be performed to understand these effects comprehensively, considering factors like dosage, skill development, past experiences, and phenomenology. Meditation experiences need to be framed within integrated longitudinal models through interdisciplinary research to develop theories and hypotheses. Ultimately, what is needed is a shift from the study of long-term meditation to advanced meditation. This investigation could lead to safer and more effective applications of meditation for improving health and well-being. 4. Glossary 4.1. Meditators Advanced meditators : Practitioners who, often through long-term practice, access deeper phenomenological states that involve a more profound awareness and transformation of cognitive and emotional processes. These meditators experience what might be called altered states of consciousness, and their practice can be characterized by mastery over cognitive, mental and emotional processes. Long-term meditators : Individuals who engage in sustained meditation practices over years. Their practice involves consistent mental training, but they may or may not necessarily reach the same depth of altered states as advanced meditators. Their experience may include mindfulness and aspects of altered awareness, but they may or may not have mastery of access the heightened states seen in advanced practitioners. 4.2. Meditation techniques Concentration, Insight and Affective-based Practices Meditation techniques can be seen as diverse approaches to mental cultivation. While some meditation techniques focus on concentration (e.g., concentration meditation, samatha), others emphasize insight into the nature of thoughts and feelings (e.g., insight meditation, vipassana), and some aim to increase empathetic and pro-social emotion cultivation (e.g., compassion meditation). This glossary is not exhaustive, and the definitions of these diverse practices are more nuanced than presented here. It is intended as an introductory guide for readers unfamiliar with the concepts discussed in the review. These terms can overlap in practice, which reflects the multifaceted nature of many meditation techniques and diverse traditions within which they have been practiced and conceptualized. Compassion meditation : This practice involves intentionally generating feelings of compassion and empathy towards oneself and others, particularly those who are suffering. During compassion meditation, practitioners may develop deep, empathetic connection with others, while also experiencing increased positive emotional feelings of kindness and compassion. This practice can lead to a heightened awareness of interconnectedness and a reduction in self-centered thinking, enhancing both emotional resilience and prosocial behavior ( Hofmann et al., 2011 ). Concentration meditation: A practice that involves focusing attention on a single object, such as the breath, to the exclusion of other thoughts or sensations. This technique cultivates a narrowing of attention, which can lead to deep states of absorption where distractions are minimized, and mental clarity is increased ( Sparby, 2019 ). Concentration meditation is sometimes called focused attention (FA) meditation. Deity meditation (with visualization) : In the Vajrayana tradition, deity meditation includes visualization of a deity or symbolic image, wherein the practitioner focuses attention on it to cultivate mental stability and concentration. It is believed to influence the practitioner's mental and emotional processes, leading to a change in perception, clarity, and the experience of self, which is thought to foster deeper self-awareness ( Eddy, 2019 ). Dharana: Dharana refers to the practice of single-pointed concentration, where attention is intentionally directed and sustained on a specific object (e.g., the breath, a sound, or a visual point). Dharana is marked by a narrowing of awareness and a deliberate effort to stabilize the mind against distraction. It is often seen as a precursor to deeper meditative states such as samadhi ( Telles et al., 2016 ). Focused attention (FA) meditation: A practice in which attention is deliberately directed toward a specific target, such as the breath, a mantra, or a visual object, while disengaging from distractions. The aim is to cultivate sustained focus and stabilize the mind by consistently returning to the chosen object when distractions arise. FA meditation strengthens attentional control and mental clarity, often serving as a foundation for achieving deeper states of absorption and concentration ( Hasenkamp et al., 2012 ; Lutz et al., 2015 ). Heartfulness meditation : A form of meditation that focuses attention on the heart or the felt experience of what is considered in this practice to be the body’s “emotional center”. This practice is thought to induce calm, experiential awareness and emotional regulation ( van’t Westeinde and Patel, 2022b ). Jhana: Refers to progressive states of mental absorption that arise from sustained, and mastery of, concentration practice. As attention deepens, practitioners experience a series of mental states wherein external distractions and internal dialogue diminish. These states are characterized by a heightened sense of tranquility, focus, and the dissolution of typical self-referential processing. In final jhana states, the practitioner experiences profound mental stillness, and awareness of subtle aspects of experience including a heightened sense of awareness itself ( Yang et al., 2025 , 2024 ). Open monitoring (OM) meditation: A practice that involves maintaining a non-reactive, open awareness of all mental and sensory experiences as they arise in the present moment. Rather than focusing on a single object, practitioners observe thoughts, sensations, and emotions without attachment or judgment, cultivating a heightened sensitivity to the flow of experience. OM meditation is associated with improved meta-awareness and cognitive flexibility, as practitioners develop the ability to monitor and let go of mental processes without engaging with them ( Lutz et al., 2015 ). Qigong: Qigong is a practice that combines physical movement, controlled breathing, and meditation. Qigong is believed to regulate the flow of what is called ‘vital energy’ ( Qi ) within the body, and is thought to promote relaxation, improve physical vitality, and enhance mental clarity. Through coordinated movements and focused breathing, practitioners aim to increase awareness of the body's internal processes, including breath and muscle tension, which can contribute to improved physical health, mental focus, and emotional well-being ( Jahnke et al., 2010 ). Rig-pa meditation : A practice within the Tibetan Dzogchen tradition, which is itself a practice within the broader Vajrayana Buddhist framework. Rig-pa refers to an awareness that is non-conceptual and is thought to directly perceive reality without mental interference. The experience is one of open awareness, where the practitioners observes without attachment or involvement of mental constructs ( Louchakova-Schwartz, 2013 ). Rinzai Zen : A school of Zen that incorporates a variety of practices, including breath awareness, seated meditation, and intensive koan training. Koans are paradoxical questions or statements designed to disrupt habitual thought patterns, leading to moments of cognitive insight and sudden shifts in perception ( Sogen, 2020 ). Samadhi: Samadhi is a state of deep meditative concentration where the practitioner's attention gradually becomes fully unified with the object of focus, leading to a complete lack of distractions. From a phenomenological perspective, this state is described as a state of heightened awareness, where self-referential thoughts diminish, and a profound sense of equanimity arises. This state is characterized by deep concentration, reduced cognitive load, and enhanced meta-awareness. In the context of this review, it is classified as a concentration-based practice ( Shankman, 2008 ). Samatha meditation (can be translated as calm meditation ) : Samatha meditation focuses on concentrating on a single object, such as the breath, and can lead to mental calm and clarity. The experience involves a steadying of the mind, where distractions are gradually quieted, enabling deeper concentration ( Harvey, 2015 ). Samatha can be practiced alongside concentration meditation, to develop mental focus. Soto Zen : A school of Zen that places a strong emphasis on shikantaza , or "just sitting," an open monitoring (OM) practice. In shikantaza , practitioner observes thoughts and sensations without judgment. This induces a state of non-dual awareness, where self-referential thinking is diminished. While shikantaza is central to Soto Zen , the tradition also values the integration of meditative practice into everyday activities ( Heine, 2015 ). Transcendental meditation : A technique involving the silent repetition of a mantra that allows practitioners to enter a state of relaxed, non-dual awareness. Transcendental meditation may lead to a mental state where the sense of self is diminished, which can include deep relaxation and reduced mental activity ( Wahbeh et al., 2018 ). Vajrayana meditation : A key aspect of the Tibetan, or Vajrayana, Buddhist tradition, Vajrayana meditation includes a wide range of meditation practices, including visualizations, mantra recitation, and focusing on symbolic imagery or deities, among many other diverse practices. These practices are thought to transform ordinary cognition and emotional states into more refined awareness ( Upadhyaya, 1988 ). In the context of this review, Vajrayana practitioners focused on deity meditation and rig-pa meditation, described above. Vipassana meditation: A Theravada Buddhist practice focused on observing the changing nature of thoughts, emotions, and sensations as they arise and pass away. This technique may enhance moment-to-moment awareness and is thought to provide insight into the impermanent and interdependent nature of mental processes ( Chiesa, 2010 ). In this manuscript, vipassana is categorized as an ‘insight’ practice, which is sometimes called open monitoring (OM). Yogic meditation : A set of practices derived from the ancient Indian philosophy of yoga, emphasizing physical postures ( asanas ), breath control ( pranayama ), and meditation. The practices focus on integrating physical, mental, and emotional processes, promoting self-regulation and greater awareness of bodily and mental states ( Elson et al., 1977 ). Zazen : The core meditation practice in Zen Buddhism, encompassing diverse techniques across traditions. Practitioners sit in silence and stillness, either focusing on the breath or simply observing thoughts without engaging with them. The practice is thought to lead to heightened present-moment awareness and mental stillness. Zazen encompasses approaches such as shikantaza in Soto Zen and the use of koans in Rinzai Zen, explained above ( Zhu, 2005 ). Highlights. Long-term meditation increases heart rate variability, improving autonomic function Meditation increases sympathetic or parasympathetic activity, based on key factors Meditative engagement is related to increased cardiac-respiratory coupling Advanced meditation is related to irregular cardiac and respiratory patterns Cardiac spectral power peaks may relate to long-term and advanced meditation Funding sources Dr. Sacchet and the Meditation Research Program are supported by the National Institute of Mental Health (Project Number R01MH125850), Dimension Giving Fund, Tan Teo Charitable Foundation, and additional individual donors. Footnotes Declaration of competing interest The authors have no competing interests to declare. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. 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