SEATTLE (AP) — High school students are getting more sleep in Seattle, say scientists studying later school start times.

Teenagers wore activity monitors to find out whether a later start to the school day would help them get more sleep. It did, adding 34 minutes of slumber a night. They also reported less daytime sleepiness, and grades improved.

The Seattle School District changed from a 7:50 a.m. start time to 8:45 a.m. in the fall of 2016 for high schools and most middle schools, joining dozens of other U.S. school districts adopting later starts to help sleep-deprived teens.

Teenagers’ nightly sleep has decreased and most adolescents don’t get the recommended nine hours. One culprit: Light from devices that many teens use to chat, post and scroll long after dark.