Delaying start times by nearly an hour has allowed Seattle high-school students to average more than a half-hour of extra sleep on school nights, a research team has found.

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When Seattle public schools adopted new start times in fall 2016 to give high-school students an extra hour of sleep in the morning, some worried that teenagers would simply use it as an excuse to stay up later.

Now, a new study that examined the sleep patterns of students at two Seattle high schools shows the policy seems to be working — leading students to get about 34 extra minutes of sleep every night. There’s even some evidence that it’s improving grades and attendance.

Teens “are not sleeping in because they’re lazy,” said University of Washington biology professor Horacio de la Iglesia, a senior author of the study published in Science Advances. Rather, they’re answering the demands of their own shifting biological clocks, which tell them to stay awake later in the evening — and wake up later, too, if they can.

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But when high school starts early in the morning, “the only thing we’re doing is chopping off their sleep,” de la Iglesia said. “Their biology is not going to change, so we might as well change what we do around their biology.”

Why does it matter? Chronic sleep deprivation is bad for physical and mental health, and it can also affect learning and memory consolidation. But later start times are difficult for school districts to pull off because they have a domino effect on logistics across the schools.

In fall 20176, Seattle became the largest school district in the country to shift start times for high schools by nearly an hour, from 7:50 a.m. to 8:45 a.m. But elementary school students had to report to school earlier than they had to before the change, to accommodate bus schedules.

Before the change took effect, the scientists went to work, using wrist and activity monitors that collect data on light and movement. Previous research has relied on students’ self-reported sleep times; the monitors, used for the first time in a study such as this one, made this research more accurate, de la Iglesia said.

With help from willing high-school students in biology classes at Franklin and Roosevelt high schools, the researchers collected data from 92 sophomore students in spring 2016, before the start-time change, for two weeks. Seven months later — when start times had moved ahead by 55 minutes — researchers recruited a second group of 88 students and repeated the experiment. (There are nearly 13,000 high-school students in Seattle public schools.)

After the start-time change, the teens had a net increase in sleep, on school days, of 34 minutes. So, no, they’re not getting a full 55 minutes extra of sleep, de la Iglesia said. But it’s definitely an improvement.

Researchers also found a slight increase in academic performance and attendance, at least in the same biology class where students were recruited to participate in the experiment, although they said it was hard to attribute the better grades and attendance to increased sleep. Final grades in the course were 4.5 percent higher overall, and tardies and first-period absences at Franklin dropped (although there was no change at Roosevelt).

Parent surveys showed there was support for the time change, and many Seattle parents seem to have adjusted to the new start times, said Seattle schools spokesman Tim Robinson. “The change has become part of our routine, so we rarely are hearing about concerns,” he said. Occasionally, parents do complain about traffic and the impact on staff commutes, as well as the challenge of scheduling sports activities, he said.

Even with the change to later start times, the teens in the study still weren’t getting the 8 1/2 to 9 hours of sleep their bodies need, de la Iglesia said. Parents can help by encouraging their teenage children to put their phones or other digital devices down earlier in the evening, to help the body’s natural sleep regulators kick in at the right time.

De la Iglesia said he hopes the research informs the debate in the Puget Sound area and across the country. “Even within the Seattle suburbs, schools have been reluctant to move to later start times,” he said.

Seattle high school start times changed in fall 2016. An earlier version of this story gave the incorrect date.