October 31, 2017
3 min read

Later school start times may lower risk for adolescent overweight, obesity

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WASHINGTON — Inadequate sleep might contribute to excess weight gain in adolescents, but delaying school start times may alleviate the risk, according to a speaker here.

“One of the most identifying contributing factors contributing to teen sleep insufficiency is early high school start times,” Rachel Widome, PhD, MHS, associate professor in the division of epidemiology and community health at the University of Minnesota School of Public Health, said during her presentation. “Based on the accumulation of evidence, the American Academy of Pediatrics and other organizations have issued policy statements recommending that high school should start no earlier than 8 a.m.”

According to Widome, past studies have shown that when schools start later, adolescents are able to get more sleep, and for every minute later a school started, adolescents would get about 30 to 90 seconds of additional sleep.

“We have seen from these studies of high schools shifting [start times] that there have been multiple benefits, including driving safety, academic performance, school attendance and mental health,” Widome said. “An interesting thing is that adolescents from varying racial, ethnic and socioeconomic backgrounds have experienced significant beneficial outcomes and increases in sleep when their high schools have delayed start times.”

Multiple factors play a role in inadequate sleep among adolescents.

“They relate to society, social environment and the adolescent brain,” Widome said. “There are two body systems that regulate sleep, and they both undergo some important changes in puberty.”

According to Widome, the first is the adolescent circadian clock.

“The release of melatonin shuts off 2 hours later relative to childhood timing around the time of puberty, and this persists through late adolescence,” she said. “After this shift, it actually becomes quite difficult for adolescents to fall asleep prior to 11 p.m. or wake up before 8 a.m.

“This is a brain shift, it’s not about them turning off their cell phones, it’s not about their friends telling them to stay up. This is something that is part of development,” Widome said. “Also, sleep drive tends to build up more slowly at the onset of adolescence.”

Still, some research gaps remain. Limited health outcome measures have been compiled in these school start time studies, Widome said.

To measure some of the health outcomes, Widome and colleagues launched the Start Study in spring 2016, a 4-year study that includes five high schools and nearly 2,400 students who were ninth graders at baseline.


“We’re interested in looking at how a shift to later high school start time relates to objectively measured weight change over time, and we’re also interested in identifying relationships between school start time and other behaviors, like physical activity and eating, over time,” Widome said.

At baseline, all five schools had an early start time. In spring of 2017, two of the high schools switched to a later start time while the other three kept the early start time. Participants completed surveys to report on sleep duration and its effects; their weight and cardiovascular parameters were measured objectively.

“As you might expect for year 1, when all participants had early start times, very few of our students were reporting optimal weekday sleep, only 15%,” Widome said. “About half of our students reported needing help waking up. Many of our students reported being tired throughout the day, and about 30% reported they were tired enough to fall asleep in class.”

Participants who slept for more than 6 hours per night were more physically active than those who slept less than 6 hours, at baseline.

“We had a quick look at 2-year data, we just got it this week,” Widome said. “When we looked at it preliminarily from year 1 to year 2, we’re not seeing an association between later-starting schools and increased physical activity, but again, these were very preliminary analyses, and just from year 1 to year 2. Hopefully, we have more data on that soon.”

Participants who reported shorter sleep durations were more likely to report feeling hungry frequently, whereas those who reported sleeping longer were more likely to report eating dinner with their families, eating breakfast and having generally healthier eating behaviors.

“We know that the jury is still out on start time and physical activity, but we do know that there are health benefits to later school start times. Nonetheless, the vast majority of U.S. high schools are still starting before 8:30 a.m.,” Widome said.

“Why have so few high school gone late?” Widome asked. “It’s not just about the scientific evidence; it’s really hard to change high school schedules. There are all sorts of built-in structures in people’s schedules that are hard to move around. Change is just hard.” – by Amber Cox


Widome R. School start times and physical activity: Impact of a natural experiment, evaluating innovative policies to reduce obesity. Presented at: ObesityWeek 2017; Oct. 29-Nov. 2, 2017; Washington, D.C.

Disclosure: Widome reports no relevant financial disclosures.