Teenagers can't help sleeping late.© PunchstockWe all know that teenagers hate to get up in the morning. But are they really just lazy, or is there a biological cause?
A European survey of the sleeping habits of 25,000 people now provides powerful evidence that biology is indeed to blame. Whereas children sleep later and later as they get older, we undergo an abrupt shift at age 20, after which we start sleeping earlier again.
The change is so sudden that researchers suggest it should be used to officially mark the end of adolescence.
We all go through phases of puberty and adolescence before we reach adulthood. Both periods begin when the reproductive system starts maturing.
Scientists agree that puberty ends when bone growth stops - at around 16 years in girls and 17.5 years in boys. But the end of adolescence - a concept that is part social, part psychological and part physiological - has always been less well defined.
Owls and larks
Our sleep and wake phases are regulated by an internal body clock. Each cycle runs at about 24 hours, but the exact timing varies from individual to individual. People with particularly early cycles, called "larks", tend to go to bed early and wake up early, whereas those with a late cycle ("owls") tend to be more active at night and sleep later in the morning.
“Do teenagers sleep late because they go to the disco, or do they go to the disco because they sleep late?”
Till Roenneberg
Chronobiologist from the University of Munich, Germany
To investigate how this "chronotype" varies throughout life, Till Roenneberg from the University of Munich, Germany, and his colleagues asked 25,000 people, aged between 8 and 90, a series of questions about what time they go to sleep and wake up.
From this, the researchers calculated the average "mid-point" of each person's sleep - in other words, the time half way between when they go to sleep and when they wake up - on days when they had no work obligations.
When they plotted the mid-points against each person's age, the researchers found that children tend to sleep later and later as they get older until the age of about 20. At that age, there is an abrupt shift in sleeping habits, and the sleep mid-point starts getting earlier and earlier again.
Roenneberg says he can't prove that the shift is caused by behavioural or environmental factors. "Do teenagers sleep late because they go to the disco, or do they go to the disco because they sleep late?" he asks.
But he believes that the abrupt timing of the shift suggests a biological cause. "This is the first time that we have been able to trace the end of adolescence," he says.
He points out that the timing of the shift also reflects the general trend of females to develop earlier than males. The women in the study that slept latest were 19.5 years of age, whereas the men's sleep got later and later until 20.9 years.
Disease clues
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"An objective marker like this could be very helpful in a range of disease conditions," says Russell Foster, a chronobiologist at Imperial College, London. Mental disorders such as schizophrenia and bipolar diseases are often accompanied by delayed sleep phases, he points out.
He suggests it could be worth investigating whether changes in the timing of this shift could be used to spot the development of such disorders early on.
Chronobiologist from the University of Munich, Germany
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