SLEEP REGULARITY IS A MAJOR KEY, SOCIAL JETLAG A MINOR, TO MENTAL WELLBEING

Human behaviour is characterized by timing and regularity of activities. On the one hand, chronotype indicates the local clock time which coincides with the midpoint of the longest (usually night-time) sleep period of the day, so that the later it is, the more evening type of person the individual is. On the other hand, diurnal (only seldom nocturnal) preference indicates the most convenient schedule for time usage. Usually, people tend to time their daily activities following local time indicated by clocks. Individuals who prefer being more active in the morning hours lie on the one end of this dimension, whereas on the other end of this dimension are those who prefer being more active in the evening hours. Thinking beyond the phenotype seen by the naked eye may deepen our understanding of the mechanisms of action which contribute to the diurnal preference or chronotype we have. It might open, if not an avenue, but a view on the actions by which mental health can be promoted. Chronotype is deduced from the timing of the longest sleep period, whereas the diurnal preference estimates the daily rhythm of feeling at one’s best from schedules for sleep, dietary intake, physical activity and cognitive exercise. Further, circadian misalignment, also known as social jetlag, is characterized by great differences in bedtimes and wake-up times from weekdays to days off and back again, as the longest sleep period is mistimed relative to the phase of the circadian rhythms. However, chronotype, circadian misalignment and diurnal preference reflect the same physiological basis of human behaviour, that is the intrinsic clock (i.e. the circadian pacemaker) which consists of the suprachiasmatic nucleus in the anterior hypothalamus and generates and maintains the circadian rhythms.