As teenagers have long complained, it's not that they're lazy, it's just that parents and schoolteachers don't understand them.

Now they have been given top-grade academic ammunition to hurl at annoying adults before slamming the bedroom door and going back to bed - lying in bed for hours at the weekend is part of their inborn cycle of sleep and wakefulness. It's circadian rhythms innit.

A US study published today in the journal Paediatrics found that early school starts were forcing students to perform academically at a time of day when they were at their worst.

The researchers, from Northwestern University in Illinois, go so far as to describe the school day as causing an "epidemic of sleep deprivation among adolescents". They say the weekend lie-in enjoyed by so many young people is simply part of their natural cycle and should not be dismissed as laziness.

Sixty students at Evanston Township high school in Illinois kept sleep diaries during school holidays and term-time as part of the study. Bright light was "administered" in the morning (presumably by flinging open the curtains?) in an attempt to normalise the timing of the circadian cycle. This made no difference at all.

They showed that adolescents lost as much as two hours of sleep per night during their first week back at school week after the summer holiday and made up at weekends by sleeping longer.

All the students performed better in class during the afternoon than in the morning, observed the researchers who gave them computer and paper and pencil tests. The young people reported being wearier, less alert and having to make a greater effort in early morning classes.

The researchers said that potential solutions to the problem included changing school start times and having tests later in the day.

They said that doctors, parents, teachers and teenagers all needed to be aware of adolescents' lifestyle patterns and their different sleeping habits.

"Knowledge of adolescent circadian rhythms 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 behaviour," the researchers said.

Early-morning light treatments did not modify the amount of sleep each student got each night, their mood, or the results of computer-administered vigilance tests.

All students performed better in the afternoon than in the morning. Students in early morning classes reported being wearier, being less alert, and having to expend greater effort.

The North Western researchers conclude sternly: "The results of this study demonstrated that current high school start times contribute to sleep deprivation among adolescents. Consistent with a delay in circadian sleep phase, students performed better later in the day than in the early morning.

"Both short-term and long-term strategies that address the epidemic of sleep deprivation among adolescents will be necessary to improve health and maximize school performance."