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.