melatonin causes it

But when hormonal changes kick in at the start of adolescence, she told AFP, young people start to stay up later and -- given the chance -- wake up later too.

Most of the students in the study were such "evening persons," she said.

"Teenagers find that they are most alert in the evening and do not feel sleepy until later, and so find it difficult to get enough sleep during school term," she added.

The key is melatonin, a hormone that signals to the body that it needs rest and sleep. In teenagers entering puberty, it is released later and later in the evening.

There are also environmental factors that contribute to the problem, she said.

Ambient light tends to minimize the amount of melatonin secreted, and the constant use of computers could keep adolescents up past their natural bedtime, even after lights are turned out.

"One thing parents can do is to lower the lights, and switch off computers and televisions an hour before bedtime," advised Warner.

In the study, Warner and two colleagues compared the sleep patterns of 310 students during a school term and while they were on holiday.

Whereas the adolescents slept more than nine hours during the school breaks, they averaged less than eight hours when hitting the books.