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Rather, it's a combination of being night owls and then suffering daytime drowsiness, researchers report.
The solution is starting school later, according to study lead author Dr. Judith Owens, director of sleep medicine at Boston Children's Hospital.
"School start times which coincide with sleep needs, amount and timing, and reduce daytime sleepiness, are critical for adolescent health, safety and performance," she said.
The findings suggest that the sleep time misalignment and daytime sleepiness associated with early school start times "may contribute to risk-taking behaviors, poor control of emotions and impaired thinking skills," Owens said.
The American Academy of Pediatrics recommends starting high school at 8:30 a.m. or later, she noted.
"Due to changes in circadian rhythms [the body's internal clock] coinciding with puberty, most teens cannot fall asleep much before 11 p.m., thus their morning wake time should be around 8 a.m., to allow them to get both an optimal amount and timing of sleep," Owens said.
Both insufficient and misaligned sleep have consequences for physical and mental health, including increased risk for obesity and depression, Owens said. These factors can also increase the chances of car accidents, sports injuries, absenteeism and lower grades, though the study did not prove that sleep problems cause those health and safety issues.
"It's not how long you sleep that has the biggest impact, but when you sleep in relation to the body's natural circadian rhythms and how impaired you are by sleepiness," she said.
For the study, Owens and her colleagues conducted an online survey of more than 2,000 students in grades 7 through 12, from 19 schools in Fairfax County, Va. The participants were asked about how long they slept, whether they were sleepy during the day, when they got up in the morning and whether they were night owls or morning larks.
The researchers also measured the students' emotional behavior and thinking skills, and took into account socioeconomic and mental health conditions.
The investigators found that about 22 percent of the teens slept less than seven hours on school nights. But, it wasn't how long they slept that affected their emotional behavior and thinking skills -- it was being sleepy during the day coupled with being up late at night.
The findings were published online Nov. 3 in the journal Pediatrics.
One sleep medicine specialist explained the connection between sleep and self-regulation during the day.
"The circadian rhythm is an internal body clock that affects physiology and metabolism in our bodies," said Dr. Sujay Kansagra. He is director of the pediatric neurology sleep medicine program at Duke University Medical Center in Durham, N.C.
One important role of the circadian rhythm is to manage sleep-wake cycles, said Kansagra, who co-authored an accompanying journal editorial.
"When it comes to the effects of sleep and sleep physiology on daytime functioning, we have to look at more than just the number of hours we sleep," he said.
Other important factors also play a role, such as where your internal body clock is set and how sleepy you feel during the day, he noted.
Teens naturally have a preference for being night owls, which may hurt their thinking abilities and emotional control during the day, Kansagra suggested.
"One important modifiable change that adolescents can make is to avoid excessive light from TVs, laptops and smartphones late at night, because light exposure late makes it more likely that the body clock will shift to a later time point," he said.
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