The American Academy of Pediatrics recommends that schools start later than 8:30 a.m. to help teens avoid becoming chronically sleep-deprived and exhausted. Adolescents need between 8.5 hours and 9.5 hours of sleep each night, but their natural sleep rhythms make it hard for them to fall asleep before 11 p.m., according to the academy.

Research has shown that teens who get too little sleep are more likely to be overweight or depressed, and they are more likely to perform poorly in school and to experiment with tobacco, alcohol and other drugs.

The CDC, in its new report, calls insufficient sleep among the nation’s teenagers a “substantial public health concern,” and says that while parents can help by teaching their children good sleep hygiene (no cellphones in the bedroom, for example), it is important for schools to do their part by ensuring that class doesn’t start too early.

“Getting enough sleep is important for students’ health, safety, and academic performance,” said Anne Wheaton, a CDC epidemiologist and the report’s lead author. “Early school start times, however, are preventing many adolescents from getting the sleep they need.”