‘Puberty’ and ‘adolescence’ are not synonyms, although both terms describe that awkward age between childhood and adulthood. Puberty is defined as the period during which the reproductive system matures. It has a clearly defined marker for when it ends: when bone growth ceases. Adolescence, by contrast, is part physiological, part psychological, part social construct. Chronobiologists joke that people suffer adolescence twice — once themselves, and again when their own children hit the teenage years. But, frustratingly, they have not been able to define precisely when it ends.


Till Roenneberg et al. now provide a suitable marker (Curr. Biol. 14, R1038–R1039; 2004). Their questionnaire-based survey of the sleeping habits of 25,000 people between the ages of 10 and 90 confirms the normal distribution of chronotypes in the population: ‘owls’, who go to sleep late and wake up late; ‘larks’, who go to bed early and wake early; and those who fall between the two (the majority of people).

The authors' survey also shows a remarkably robust relationship of this chronotypology to age (see picture). Children are typically early risers but start to sleep progressively later as they enter adolescence. They reach maximum lateness at around the age of 20, when the curve abruptly starts to decline. The normal distribution of owl-ness and lark-ness in the population is apparent at each age — but, the authors suggest, it is the point at which an individual's curve inverses that marks the end of adolescence.

The data also reflect the earlier development of females compared with males; young women reach their maximum lateness at 19.5 years and young men at 20.9 years. Females tend to be earlier chronotypes than men throughout adulthood, but the gender difference disappears at around 50 — the onset of menopause.