WHO, WHAT, WHY?
The Magazine answers...
Parents need no reminding of how late and long teenagers can sleep. Is there a biological reason why adolescents struggle to get up?
'It's my body clock, mum!'
The spotty teenager emerging from the bedroom at midday is a common sight in any family household, but until now it was presumed the habit was linked to young rebellion. Or laziness.
Now researchers believe there could be biological reasons why those aged 10 to 20 need to sleep later and longer than the rest of us. About nine hours, to be exact.
Tests by Professor Russell Foster, chairman of circadian neuroscience at Brasenose College, Oxford, suggest that students perform better in the afternoon, because their body clock is programmed about two hours later, possibly for hormonal reasons.
"There's a biological predisposition for going to bed late and getting up late. Clearly you can impose upon that even worse habits, but they are not lazy."
By delaying the start of school by one hour, and moving more demanding subjects to later in the day, then absenteeism and depression will fall, he says. But that does not mean young people should be indulged and allowed to stay in bed late.
"There's a responsibility for teenagers and their parents to know that for full cognitive performance, you need a good eight to nine hours.
"You need to count back nine hours from the alarm clock going off and you need to make sure you are wound down and ready for bed, and not playing video games and watching television."
So why are teenagers different from the rest of us?
Neil Stanley, a sleep researcher at the University of East Anglia, says: "There's a blip in teenagers where they need to have more sleep, but also their timing of that sleep is shifted so they want to go to bed later and get up later in the morning.
"While that is what we know, we don't know why it is. There must be an evolutionary reason why this happens."
WHO, WHAT, WHY?
A regular part of the BBC News Magazine, Who, What, Why? aims to answer some of the questions behind the headlines
It could be linked to the "crisis" of the teenage years - the body changes, the exams, the leap into adulthood, and functioning without emotional support.
"If sleep is important for memory and learning, and dealing with emotions, and repair and recuperation, then teenage years have an awful lot of that. So that might explain the increased need for sleep, but it doesn't explain the change in timing of sleep."
After the age of 20, sleep becomes lighter as we get older, but the time we need to sleep each night remains fairly constant.
It's not just about biology, says Jim Horne, director of Loughborough University's Sleep Research Centre. Besides the body clock, there are several factors that make teen sleep patterns distinctive.
"In puberty the brain undergoes a bit of reorganisation and sleep provides the opportunity for the brain to reorganise itself."
The time shift forward could also be explained by simple social issues like young people trying to stay up later than their parents or socialising late, he says.