Melatonin and serotonin related characteristics in morning-type and evening-type individuals

Background: Morningness-eveningness, also termed „chronotype“, is an individual characteristic that refers to the time-of-day-dependent preference of sleep and activity. These chronotypes are attributed to differences in the circadian clock. This study examined whether the chronotypes differ in the onset timing of melatonin secretion (mel-on), as well as in the slope up of mel secretion and in platelet 5HT concentration. Methods: Secretion profiles of mel were studied in blood samples of 50 entrained, healthy subjects (20 males/30 females, aged 18–35 years) with a regular sleep schedule. The subjects underwent a constant routine with dim light (<10 lux) between 19:00h and 22:30h. Additionally, mel concentration in the morning at 9:00h and platelet 5HT content were analysed. Results: Based on the original criteria of the Horne and Ostberg Morningness/Eveningness Questionnaire, the sample included 8 evening (E) types, 27 nether types and 15 morning (M) types. The individual time intervals between the dim light onset (19:00h) and mel-on were very variable, being very short in M-types (30min) and very long in E-types (>180min). Moreover, M-type and E-type subjects were significantly different in the slope up of mel concentration, as well as in their platelet-5HT concentration. Conclusion: The phenotype variability of morningness/eveningness in healthy subjects seems to be associated with a predisposition to very early or very late initiation of mel production and secretion.