Enter puberty. Puberty delays the circadian clock, meaning teens are naturally wired to stay up later. During this time, Hasler said, it takes longer to fill the sleep drive, feel tired and want sleep.
Children get sleepy soon after sunset, but most teens — especially boys — prefer to stay awake about three hours longer and sleep in later the next day. Then around the age of 20, our schedules creep back again toward being morning larks. This same pattern holds in other mammals. But why?
“The evolutionary perspective has been that sleep-phase delay allows adolescents to pursue autonomy, and animal researchers put it towards competition for mates,” Hasler said. “By staying up later, they’re not competing with parents for reproductive success, which gives an advantage.”
In other words, in the days of primeval humans, adolescents might have used this trait to compete with older alpha males and females who slept earlier, given that heightened sexual activity tends toward “eveningness” in animals as well as men and women today.
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