If you think school starts too early in the day, you’re not alone. Experts have long argued for later start times in middle and high school. A new study used activity trackers worn on the wrist to see how such a delay affected kids in a real school. And it showed kids slept more, got better grades and missed fewer days of class when their school day started somewhat later.

Adolescents are different from younger kids. Most don’t feel ready for bed until after 10:30 p.m. That’s because puberty shifts everyone’s circadian (Sur-KAY-dee-uhn) rhythms. These are the 24-hour cycles our bodies naturally follow. Among their tasks: They help regulate when we fall asleep and when we waken.

The shift in our body clocks may not be as obvious as puberty’s physical changes. But it’s just as important.

The shift is related to melatonin (Mel-uh-TONE-in), the hormone that helps us fall asleep. “When puberty begins, a teenager’s body doesn’t secrete that hormone until later in the evening,” notes Kyla Wahlstrom. She is an expert on human development and education at the University of Minnesota, in Minneapolis. She was not involved in the new study.

Even with their shifted rhythms, teenagers still need 8 to 10 hours of sleep each night. If they fall asleep late, they’ll need more snooze time in the morning. That’s why doctors, teachers and scientists have recommended for many years that school should start later.

Some school districts have listened. For the 2016–2017 academic year, the high-school start time in Seattle, Wash., changed from 7:50 to 8:45 a.m. The new study analyzed the results of that delay.

A real-world experiment

The researchers looked at sleep patterns in high school sophomores a few months before the schedule change. Then they studied the following year’s sophomores eight months after the change. In all, about 90 students at two schools took part in the study. The teachers were the same each time. Only the students differed. This way, the researchers could compare students of the same age and grade.

Instead of just asking students how long they slept, researchers had students wear activity monitors on their wrists. Called Actiwatches, they’re similar to a Fitbit. These, however, are designed for research studies. They track movements every 15 seconds to gauge whether someone is awake or sleeping. They also record how dark or light it is.

Students wore an Actiwatch for two weeks before and after the change in the school start time. They also completed a daily sleep diary. Actiwatch data showed that the new schedule gave students 34 extra minutes of sleep on school days. That made it more similar to sleep periods on weekends, when the students didn’t have to follow a set schedule.

“In addition to getting more sleep, the students were closer to their natural sleep pattern on weekends,” says Gideon Dunster. “That was a really important finding.”

Dunster is a graduate student in biology at the University of Washington in Seattle. He and biologist Horacio de la Iglesia led the new study.

The Actiwatch light-tracking showed that students didn’t stay up later after the shift in school start times. This light analysis was a new feature of the study, notes Amy Wolfson. She is a psychologist at Loyola University Maryland, in Baltimore. She didn’t work on the Seattle study. But she notes that other studies have shown that more exposure to light at night is not healthy.

Besides getting more Zzzz’s, students who could sleep in later also got better grades. On a scale of 0 to 100, their median scores increased from 77.5 to 82.0.

The study doesn’t prove that the schedule change boosted their grades. “But many, many other studies have shown that good sleep habits help us learn,” says Dunster. “That’s why we concluded that the later start times improved academic performance.”

The Seattle team published its new findings December 12 in Science Advances.

Links between snoozing and learning

Teens who don’t sleep well may find it harder to absorb new material the next day. What’s more, people who don’t sleep well also can’t process well what they had learned the day before. “Your sleep puts everything you’ve learned into ‘file folders’ in your brain,” Wahlstrom says. That helps us forget unimportant details, but preserve important memories. Every night, a fluid also flushes out molecular wastes that can damage the brain.

Wavebreakmedia/iStockphoto Tired students are less likely to learn in class. Overnight, as they sleep, they also are less likely to cement to memory what they had learned in class.

And there’s another link between sleep and grades. Kids won’t learn if they don’t make it to class. That’s why teachers and principals worry about kids missing school or being tardy.

To see if later start times affected attendance, the researchers looked at the two schools separately. One had 31 percent of students from lower-income families. In the other school, 88 percent came from lower-income families.

In the wealthier school, there wasn’t much change in missed school hours. But at the school with more low-income kids, the new start time boosted attendance. During the academic year, the school recorded an average of 13.6 absences and 4.3 tardies for the first period. Before the schedule change, those yearly numbers were 15.5 and 6.2.

The researchers don’t know what is behind this difference. It’s possible that lower-income kids rely more on the school bus. If they sleep late and miss the bus, it may be too hard to get to school. They may not own a bike or car and their parents may already be at work.

Lower-income kids sometimes get worse grades than their wealthier peers. Wahlstrom says there are many reasons why this might happen. Anything that helps reduce this achievement gap is a good thing. That includes better class attendance.

Wolfson thinks it’s fantastic that the activity trackers confirmed what sleep researchers had known for a long time. “I hope all this will have an impact on school districts around the country,” she says. “Moving school start times to 8:30 a.m. or later is an effective way to improve health, academic success and safety for adolescents.”