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PARIS (AFP) - - Scientists have confirmed what parents of teenagers have always suspected: adolescents are out of sync with the rest of the world.

Most teens probably don't get enough sleep and suffer in their school work because their internal clocks make them night owls, according to a study published Tuesday.

Researchers in Australia showed the average teenager misses more than an hour of sleep each night and is forced to wake up 2.5 hours earlier than his or her natural rhythms would dictate.

High school students with a late-night "circadian preference," as the biologically-driven cycle is called, reported doing more poorly in school, and feeling more frequently depressed and unhappy.

"For all people, there is a genetic disposition to being either a 'morning lark' or a 'night owl'," explained lead author Suzanne Warner, a professor at Swineburne University of Technology in Hawthorn, Australia.

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.

"Night owls" were more likely than "morning larks" to have negative attitudes about themselves, to express feelings of unhappiness and voice irritation with their classmates, according to the study, published in the Netherlands-based Journal of Adolescence.

They also complained of low energy and "impaired" daytime functioning.

"For classes that start before 9:00 a.m., we have to question whether the students are going to be alert and able to learn," said Warner.

Previous research has shown that nine hours is the optimal amount of sleep time for teenagers.

Circadian clocks are found in organisms ranging from bacteria to human beings, and impose a roughly 24-hour schedule on our activities, such as sleeping and eating.

The mechanism controlling these rhythms is found in individual neurons located in the suprachiasmatic nuclei inside the brain. Scientists have identified at least one gene that determines whether one's "clock" will be naturally set for early or late rising.

The same process is involved in jet lag, Warner points out. "You could say that a lot of young people feel quite jetlagged coming into the school term -- it is a very similar feeling," she said.

Parents should rethink a tendency to let adolescents set their own bedtime schedule, she added.