Heart Rate Variability, Physical Activity and Exhaustion in the Inpatient Treatment of Stress-Related Disorders

The goal of this observational study is to learn about heart rate variability and physical activity in inpatients with stress-related disorder. The main questions to answer are: * whether heart rate variability shows the severity of certain symptoms. * whether physical activity has an influence on heart rate variability and symptoms of stress-related disorder. Participants will be asked to take part in 5 examinations during their up to 8-week inpatient treatment: * Participate in an interview * Measurement of heart rate variability, blood pressure and health of the arteries (pulse wave velocity) * Tests of endurance, strength, balance and cognitive abilities * Completion of two additional short questionnaires * Wearing a 24-hour electrocardiogram * Wearing a fitness watch and daily self-assessment of fatigue, mood and tension via an app on participant's mobile phone during their stay.