Only 20 per cent of American teens get the nine hours of sleep per night recommended by the National Sleep Foundation, according to the group's poll released Tuesday.

The poll also found that 28 per cent of high school students said they fell asleep in class at least once a week, while 14 per cent arrive late for school or miss it entirely because they sleep in.

The Sleep Foundation said people aged 13 to 22 need nine to 10 hours of sleep each night.

Dr. Manisha Witmans, of Stollery Children's Hospital in Edmonton, says there are still many mysteries about sleep's function.

The 2006 Sleep in America found that sixth graders get an average of 8.4 hours of sleep a night, but Grade 12 students get just 6.9 hours.

The poll also found that half of teen drivers said they have driven while drowsy in the past year. And 28 per cent of teens polled said they're too tired to exercise.

The Sleep Foundation's poll interviewed 1,602 parents and their children aged 11 to 17 between Sept. 19, 2005 and Nov. 29, 2005. Its margin of error was 2.4 percentage points.

Mary Carskadon, an expert on adolescent sleep patterns who led the study, and said the old adage "early to bed, early to rise" doesn't necessarily work for teens.

While school starts early for most teens, the biological trigger that causes sleepiness slows down during adolescence, so teens can't fall asleep until later at night, said Carskadon, a professor at the Brown Medical School and director of the sleep lab at the E. P. Bradley Hospital in Providence, R.I.

Sleep still a mystery to scientists

Scientists are still trying to determine the role of sleep in human biology and behaviour.

"Believe it or not, despite all that we know about sleep, we don't know why people sleep or what its purpose is," said Dr. Manisha Witmans, who wasn't involved in Sleep Foundation poll but studies sleep issues at the Stollery Children's Hospital in Edmonton.

In the first stage of sleep, the brain slows down slightly for about two hours. In the next stage, REM or rapid eye movement sleep, brain activity increases as people dream. There are only theories on what the purpose of dreaming is.

"We believe it's associated with learning and memory consolidation," said Witmans.

Part of scientists' understanding of what happens when people sleep comes from what happens when they do not get enough.

"People tend to be irritable. They have impaired daytime function," said Witmans. "There is data to suggest that people that have sleep disorders are more at risk for depression."