Delaying school start times has helped Seattle teenagers get a better night’s sleep.

During puberty, circadian rhythm is altered, and sleeping and waking are typically delayed to a later time. This creates a problem: Adolescent wake-sleep patterns do not coincide with those of conventional social life, and teenagers rarely get the recommended eight to 10 hours of sleep each night.

In 2016 the Seattle School District changed high schools’ opening time to 8:45 a.m., 55 minutes later than it had been. Using wrist monitors, researchers tracked sleep onset and duration for two weeks in 10th-graders before and after the change in two schools, one economically disadvantaged.

Before the change, students got an average of six hours and 50 minutes of sleep a night. Afterward, they got seven hours and 24 minutes. Bedtimes before and after were unchanged.

The study, in Science Advances, also found that the change was associated with a 4.5 percent increase in grades, although the researchers could not prove it was causal. They also found an increase in punctuality and attendance, but only in the economically disadvantaged school.

“Many adults have the idea that teenagers are lazy,” said the senior author, Horacio O. de la Iglesia, a professor of biology at the University of Washington. “They’re not. Kids need nine hours of sleep, and we can help them get to that goal by delaying school start times.”