Societies define adulthood in different ways, from entering puberty to entering the workforce. But circadian clock researchers now suggest that adolescence ends when we stop sleeping in.
Teenagers are more likely to have trouble getting out of bed in the morning than are young children or adults--a finding many studies attribute to a chronic lack of sleep. But researchers at the University of Munich wondered if a more fundamental biological factor played a role.
Using a brief questionnaire distributed in clinics, universities and online, Till Roenneberg and colleagues collected data on sleeping patterns from more than 25,000 people in Germany and Switzerland. As part of their analysis, the researchers determined each person's "chronotype" by calculating the mid-point of their sleep--halfway between going to bed and waking up--on days when the subjects slept as late as they wanted.
A surprising pattern emerged. Average chronotypes drift later and later during the teen years, but then begin to move steadily earlier after the age of 20, the researchers report in the 28 December issue of Current Biology. It still isn't clear why, says Roenneberg. Teenagers may sleep late because they've been out partying or they may go out because they're wide awake at 11 pm. However, he says, the team also saw a similar pattern in teenagers in rural valleys in South Tyrol--where nightclubs are relatively scarce. There, the average chronotype was about an hour earlier, but the overall age pattern was the same. The researchers also saw differences between the sexes, with females having an earlier average chronotype than males until around age 50--consistent with menopause--when the correlation between age and chronotype seems to break down. This suggests, Roenneberg says, that biological factors such as hormones have an important influence on the tendency to sleep late.
Sleep researcher Mary Carskadon of Brown University in Providence, Rhode Island, says that both social and biological factors are likely involved. Finding the biological trigger--if any--could lead to a better understanding of what drives circadian rhythms, she says.