Desmopressin Melt Therapy in Nocturnal Polyuria Patients: Pharmacodynamic Study

The objective of this study is to find out what the pharmacodynamic (PD) characteristics of desmopressin melt are in nocturia patients (compared to healthy volunteers and children). The main questions the investigators want to answer are: * Are differences related to the pathophysiological factors involved in nocturia? * Are there age/gender/size differences? * Can the investigators identify patients who are likely to develop hyponatraemia? * Can the investigators individualize treatment and reduce risk for hyponatraemia? The patient will be given a prescription to buy Minirin Melt 60µg at the local pharmacy. During 30 days, the patient has to take Minirin Melt 60µg in the evening before going to bed. There are two groups of patients: Group A: Patients that still have to undergo an evaluation phase, according to standard procedures (e.g. osmolality test, blood sample). This procedure is not a part of this study. The study starts when the 1st Minirin Melt tablet has been taken (= day 1). Group B: This group already went through the evaluation phase (by participation to study 1 or study 3 as mentioned above) and they have been prescribed Minirin Melt 60 µg ambulatory. The study starts when the patients takes his first prescribed Minirin Melt tablet: * On day 3 and day 7 a blood sample will be taken at the UZ Ghent for safety control (Na+, K+, creatinin, osmolality = good clinical practice guideline). These first 2 visits (day 3 and day 7) are standard procedure. If the patient is at high risk for side effects, blood has to be taken during the first 7 days. * On day 3 the patient has to give a urine sample. * Patients have to fill out a frequency/volume chart during the first 14 days. * On day 30, a 3rd blood sample will be taken