“I’ve gained an hour of sleep,” she said. “I definitely feel a lot better. I find myself waking up around 7:30 without an alarm because it’s a natural time. It’s a great, great feeling.”

Other Seattle high school students also are sleeping more — a median of 34 minutes a night more — since the school district pushed back the start of classes from 7:50 a.m. to 8:45 a.m. in fall 2016, a new study shows. Plus, when school began later, grades and attendance went up, and tardiness went down.

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After Franklin High in Seattle reset its starting bell, teacher A.J. Katzaroff’s first-period biology students’ median grades rose from a C to a B. “Kids were more awake, more present and more capable of engaging in intellectual work because they had the rest they needed,” she said.

Cindy Jatul, a biology teacher at Seattle’s Roosevelt High, also saw the benefits of the later start time on her students. “Prior to the change, my first-period class would just make silly mistakes because they weren’t firing on all cylinders,” she said. “They were in this kind of fog. There were kids who were sleeping in class, their heads on the table.”

After the change, students were more alert and engaged. Parents told Jatul, “I have my kids back. They’re not zombies.”

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The average American teenager is chronically sleep deprived — a condition the American Academy of Pediatrics identified as a public health issue in 2014, when it called on middle and high schools to delay the start of classes until at least 8:30 a.m. Hundreds of U.S. schools have changed to a later schedule. Still, fewer than 1 in 5 comply with the academy’s guidance, according to Start School Later, a Maryland nonprofit advocacy group.

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Why not just send kids to bed earlier? Because powerful biological forces push adolescents’ circadian rhythms later and seem to make it impossible for them to fall asleep early enough to get sufficient shut-eye to rise before dawn to get to school on time.

“Parents say, ‘It’s 10 p.m.; it’s time to fall asleep.’ But adolescents are biologically unable to fall asleep that early. Then the problem is we’re pushing them to wake up very early because they need to get to school. So you’re basically chopping off the last chunk of sleep they need,” said sleep expert Horacio O. de la Iglesia, senior author of the Seattle study and a University of Washington professor.

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“Teenagers are not lazy. They’re simply trying to sleep the amount of time they need,” he said.

Sleep deprivation can lead to depression, stress, suicidal thoughts, delinquent behavior, obesity, car crashes and poor academic performance, earlier studies have shown. Based on students’ own reports, researchers also have previously demonstrated that delaying school start times resulted in students sleeping longer and performing better academically.

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The new study published in the journal Science Advances goes a step further.

In Seattle, researchers gave 94 Franklin and Roosevelt sophomores wrist-activity monitors to collect data on their sleep when school started at 7:50 a.m. When school began at 8:45 a.m., another 84 sophomores wore wrist monitors to record their sleep.

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The monitors showed that at first, when classes started earlier, students at the two schools slept a median of six hours and 50 minutes. When classes started later, their median sleep length increased to seven hours and 24 minutes — a significant improvement, yet still more than an hour shy of the 8½ to 9½ hours a night the American Academy of Pediatrics recommends.

Nonetheless, the increase in sleep appeared to be sufficient to boost academic performance at both schools. Median first-period biology grades rose 4½ percent after the delayed start time.

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In terms of attendance, however, the change was more of a boon for students from less affluent families.

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First-period absences and tardiness dropped significantly at Franklin, where 88 percent of students are economically disadvantaged, to levels similar to those of Roosevelt, where 31 percent fall into that category. Absences and tardiness at Roosevelt dropped, but barely perceptibly.

Teachers expected the school with more economically disadvantaged students to see a greater improvement in attendance. Roosevelt students have more transportation options, Jatul said. She knows parents who hire shuttle buses to drive their children to Roosevelt — an option unavailable to the vast majority of Franklin parents.

The findings on attendance mirror findings in other studies showing that students from economically disadvantaged households are more likely to benefit from later school start times, said Mary Carskadon, a psychiatry professor at Brown University’s Alpert Medical School.

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“You miss your bus, you don’t have a parent who can drive you to school or can give you an Uber account,” she said. “So those kids will miss school.”

Carskadon, who was not involved with the Seattle study, began studying sleep as a doctoral student at Stanford University in the 1970s. Her hypothesis: As children pass through adolescence, they need less sleep.

She soon learned she was dead wrong.

Since Carskadon defended her dissertation, school start times have moved earlier and earlier at the same time that electronic devices have moved into teens’ bedrooms. “The kids are sort of trapped by their biology on the one hand and society on the other,” she said. She calls it a “perfect storm.”

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The simplest way to calm the storm is to delay school start times, public health advocates say. Yet school districts have been slow to respond to the call, largely as a result of fears about logistics.

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Changing school start times requires a number of other changes, such as altering school bus and family drop-off and pickup schedules. It often means adding time to the end of the school day, eating into the time when athletes train or compete and when students work or babysit for younger relatives.

At Roosevelt High, football practice now starts before school, leaving players complaining that they’re more exhausted than ever, de la Iglesia said.

“It’s not ideal,” Jatul conceded. But “are we going to make the majority of students sleep deprived because a certain number of athletes need to get out of school earlier?”

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Compounding the problem is that unless all competing teams change school release times, students whose schools end later will have to miss class to compete.

No state has stepped in to regulate school start times. In 2018, the California legislature did pass a bill that would have prohibited all public middle and high schools, except those in rural areas, from starting earlier than 8:30 a.m. But former governor Jerry Brown vetoed the measure, saying teachers and school boards opposed its “one-size-fits-all approach.”

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Carskadon, the Seattle teachers and de la Iglesia all stressed the importance of combining delayed school times with education about sleep.

“This needs to be paired with really good advice to parents, teachers and students about what’s inhibiting their sleep onset in the evening,” de la Iglesia said. For example, “It’s not a good idea to go to bed with a phone.”