Now this is interesting... to me, at least. Especially as a developmental follow-up on my previous post about sleep and clocks (http://sciencepolitics.blogspot.com/2004/12/sleep-repression_27.html).
It has been known that adolescents are quite extreme “owls” no matter what their chronotype may be earlier and later in life (and fortunately, school districts are starting to recognize this). This has been attributed to the surge of sex hormones in early adolescence. Responsiveness of the circadian clock to sex hormones has not been studied much (virtually not at all, though I should be able to publish my data within a year or so, sorry for not being able to divulge more detailed information yet), yet most people in the field believe this to be the case, even if no details are available yet.
Now this paper suggests that the end of adolescence should be defined as a time when the circadian clock goes back to its “normal” state. But, wait a minute, the hormones do not disappear at that time. Thus, if the clock is responding to the hormones at the onset of the adolescence, does this mean that the end of adolescence should be defined as the time when the clock becomes UNRESPONSIVE to the hormones? How does that happen and how is that triggered?
Anyway, I still have to look at the study itself (this is just a press release). I want to see if females both become “owls” AND quit being “owls” earlier than males [OK, I took a peek at the paper and yes, they do]. Also, in women, hormones (mostly estrogen and progesterone) surge in monthly cycles that end abruptly at menopause, while in men testosterone (mainly) is pretty high (with a small circadian variation) continuously and only gradually declines in old age. The lifelong sex difference they found in the study is quite interesting in this light.
Also, I like the way they tried to tease away social influences from pure biology, though they are correct to warn they do not know in which direction causation flows: do the teenagers sleep late because they party, or do they party because they are wide awake…..and now a closet sociobiologist is waking up somewhere in my head trying to explain why would it be adaptive for teens to stay up late and play, including perhaps experimentation with sex while elders are asleep (squash, bad sociobiologist…go back to sleep…there, good boy)….
Wake Me When It's Over
"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."
Of course, the study was done on Germans. Even in disco-less South Tyrol there is electricity and modernity. It would be cool to see a similar study performed in a culture where sleep is divided in two parts (late-night sleep and afternoon Siesta), like in Mediterranean and Latin American countries, as well as in a real primitive society in which sleep is divided into two parts (early-night sleep and late-night sleep with a break for sex around midnight).