New research reveals an important link between sleep and self-regulation in teenagers and suggests that later school start times might lead to better health and school performance.

The paper, just published in Pediatrics, is based on a larger study co-led by Robert Whitaker, professor of public health and pediatrics at Temple University, and Judith Owens, director of the Center for Pediatric Sleep Disorders at Boston Children’s Hospital.

Chronic insufficient sleep is at epidemic levels in U.S. teens. It is thought to be a major cause of impaired ability to self-regulate thinking, emotions or behavior, which can have harmful consequences like depression, substance abuse, accidents, and academic failure. But while many people assume that the number of hours teens sleep is the main problem, Owens and Whitaker found that the tendency to be a night owl—to go to bed late—is a better predictor of poor self-regulation.

“We were surprised to learn that it is not how long you sleep on school nights that has the biggest impact on self-regulation, but how sleepy you are during the day and when you sleep in relation to the body’s natural circadian rhythms,” says Whitaker. That’s particularly important because the circadian patterns of teenagers naturally shift toward the “evening chronotype”—the tendency to be a night owl. But early start times at middle and high schools force teens to wake up during their lowest levels of alertness—the equivalent of asking adults (who have different sleep patterns) to wake up at 3 a.m.

The researchers hope their findings will add weight to growing scientific evidence that school start times should be better matched to natural adolescent sleep patterns. “Early schools start times force many teens to be mentally alert at a time of day that is out of sync with their own circadian rhythms,” says Whitaker. “Having later school start for teens times might improve their academic performance and social functioning because it improves their self-regulation.”

The study was supported in part by the Robert Wood Johnson Foundation.