One quarter of adolescents go to bed after 11.30pm on school nights, according to a new United States study, which also finds those kids tend to perform worse in school and to have greater emotional distress than peers who go to bed earlier.
"If teens' sleep patterns are in conflict with their natural circadian rhythms, then that has repercussions on cognitive function and emotional regulation, as well as potential health consequences," said Dr Judith Owens, director of sleep medicine at the Children's National Medical Center in Washington, DC, who was not involved in the study.
Based on surveys of adolescents across the US in the 1990s, and follow-up surveys as those kids grew into adulthood, researchers at the University of California, Berkeley, analysed why teens were getting too little sleep and what the long-term effects might be.
It's estimated that between 45 per cent and 85 per cent of sixth to 12th graders sleep less than the recommended nine hours on school nights, according to the report in the Journal Of Adolescent Health. And nearly half of students - 44 per cent - report difficulty staying awake during school, the authors write.
The timing of bedtime is potentially just as important as the amount of time teens sleep because, like everyone else, they have a natural circadian rhythm.
"Adolescents really can't fall asleep much before eleven and are biologically programmed to wake up about eight," said Dr Owens.
Dr Owens, a professor of paediatrics at George Washington University School of Medicine and Health Sciences, said adolescents typically need about nine hours of sleep in order to perform at optimal levels.
She advises parents to set sleep rules so that teens get about nine hours of sleep every night, with bedtimes that are similar each night, including on the weekends.
Naps aren't a substitute for a good night's sleep but a short afternoon nap can help recharge a teen's batteries, she added.