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A group of young people get some sleep during the World Youth Day Festival last year
© AFP/DDP/File Jochen Luebke

The problem could even be fatal, as adolescents learn to drive often without enough sleep, the study said.

Only 20 percent of children aged 11-17 get the nine hours' sleep recommended during the school week, while 45 percent get less than eight hours, according to the National Sleep Foundation survey.

Nearly 30 percent of adolescents doze off in class at least once a week and 14 percent are regularly late to classes because they over-sleep.

Sleep-deprived students are more likely to get poor grades, while 80 percent of those getting a good night's sleep report getting good or excellent grades, the study said.

The survey contacted 1,602 families by telephone, September 19 to November 29, 2005. Ninety percent of parents were not aware that their children were behind in their sleep.

"This poll identifies a serious reduction in adolescents' sleep as students transition from middle school to high school," said Sleep Foundation President Richard Gelula.

"This is particularly troubling as adolescence is a critical period of development and growth -- academically, emotionally and physically."

He added: "At a time of heightened concerns about the quality of this next generation's health and education, our nation is ignoring a basic necessity for success in these areas: adequate sleep."

He urged parents, educators and teenagers "to take an active role in making sleep a priority."

The survey noted that a child's circadian rhythms, their bodies' internal clocks, shift when children enter adolescence, toward feeling more awake at night and toward awakening later in the morning.

This natural tendency makes it difficult for adolescents to sleep before 11:00 pm, according to the survey, which showed more than half of US students go to sleep at that hour or later during the school week.

The study also shows that nearly all US high school students arise at 6:30 am or earlier to get to school by 7:30 am, the usual start time for the school day that ends at 2:30 or 3:00.

"A trick of nature, this 'phase delay' can make it difficult for them to fall asleep before 11:00 pm more than one-half (54 percent) of high-school nights," said the co-chair of the foundation's sleep and teen task force, Jodi Mindell.

"However the survey finds that on a typical day, adolescents wake up around 6:30 am to go to school leaving many without the sleep they need."

"In the competition between the natural tendency to stay up late and early school start times, a teen's sleep is what loses out," she said.

"Sending students to school without enough sleep is like sending them to school without breakfast," she said.

While sleep serves a restorative function for adolescents' bodies and brains, it also provides a time to process what has been learned during the day, she added.

However, catching up on sleep during the weekend is not an answer, said Mary Carskadon, who directed the research.

"Irregular sleep patterns that include long naps and sleeping in on weekend negatively impact adolescents' biological clocks and sleep quality, which in turn affects their abilities and moods," she said.

Keep it simple, she said. "Many teens have a technological playground in their bedrooms that offers a variety of ways to stay stimulated and delays sleep."

Caffeine disrupts sleep cycles, too said the study, which found three-fourths of adolescents consume at least one caffeinated drink daily, while nearly one-third drink two or more.

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