Teenagers must have that lie-in

It's the excuse that every teenager has been looking for. They cannot help sleeping until noon at the weekend - it's the way they are made.

Researchers studying the sleep patterns of adolescents found that the weekend lie-in is part of a child's natural cycle and is not laziness.

It is even more marked in children who have to start school early on weekdays and so have to perform academically when they are at their worst, says the US study, published today in the journal Pediatrics.

The scientists say that people who deal with teenagers - paediatricians, parents, teachers and other teenagers - should be aware of their sleep patterns.

"Knowledge . . . could promote better family relationships if parents understood that sleeping late on weekends is part of their children's in-born cycle and not lazy or antisocial," they said.

Students at schools in Illinois kept diaries showing that most lost up to two hours of sleep a night during the week but at weekends slept like they did in the holidays.

All the students performed better in the afternoons than in the mornings and pupils in early morning classes reported being wearier and less alert.