A marker for the end of adolescence

Between childhood and adulthood, we go through puberty and adolescence. While the end of puberty is defined as the point of cessation of bone growth (epiphyseal closure; girls: 16 y; boys: 17.5 y), the end of adolescence (∼ ∼19 y) is defined less clearly, by a mixture of physical, psychological, social, and mental measures [1]. One conspicuous property of adolescence is the apparently unsaturable capacity to stay up late and to sleep in. Investigating 'chronotypes' we observed an abrupt change in the timing of sleep at around the age of 20 and propose this change as the first biological marker of the end of adolescence. Chronotype refers to when an individual's endogenous circadian clock synchronises (entrains) to the 24 h day. When shielded from environmental time cues, i.e. cycles of light and darkness, circadian clocks 'run free', with a period of about one day (hence 'circa-dian'). In nature, they are entrained to the earth's 24 h light–dark cycle [2], with individual clocks entraining differently — some very late ('owls'), others very early ('larks'), and the majority in between. The chronotype is predominantly defined via behavior, e.g. activity onset in animals which strongly correlates with sleep offset. To assess the distribution of chronotypes in the population, we use a simple questionnaire (Munich ChronoType Questionnaire, MCTQ [3]) that asks people about their sleep habits on work and free days; due to the open design of our study, the age distribution is not precisely representative (Figure 1A). The reference point to assign chronotype is based on the midpoint of sleep on free days (MSF, i.e. sleep without social obligations). Because most chronotypes tend to accumulate a sleep debt on work days, which is compensated for on free days [3], MSF is adjusted for individual average sleep need (MSF sc ; see Supplemental data). Figure 1B shows the distribution of chronotypes (MSF sc) in our database (N ≈ ≈ 25,000; mainly from Germany and Switzerland). The chronotype depends on genetic [4,5] and environmental factors [3] but also on age [3,6–9]. Within each age group, the distribution is very similar to that of the general population (Figure 1B); the respective means, however, vary systematically (Figure 1C). Children are early chronotypes and become progressively later (delaying) during development, reaching a maximum in their 'lateness' at around the age of 20. After 20, they become earlier again (advancing) with increasing age. The general tendency of females to …