Almost three-quarters of American teenagers get less than the recommended eight to 10 hours of sleep on most nights.
That’s not healthy — for a whole host of reasons. As the National Sleep Foundation points out, not getting enough sleep can trigger overeating, particularly of sugary and other unhealthy foods. It can also interfere with a teenager’s ability to concentrate in the classroom and contribute to impatient, aggressive behavior with peers and teachers. Some research has even suggested that lack of sleep worsens acne and other skin problems.
Many studies have reported that people who exercise regularly tend to have longer and better-quality sleep — even after just a few days. But those studies have mostly involved adults.
Researchers at Pennsylvania State University decided to fill that research gap. In a study whose results were published Wednesday in the journal Scientific Reports, they followed several hundred teenagers for a week to see what effect physical activity had on their sleep patterns.
Conversely, on days when the teenagers were more sedentary than usual, they fell asleep later and slept for a shorter length of time.
These findings — that exercise has a positive effect on sleep and sedentary behavior has a negative one — is not unexpected, given all the previous research on this topic. What is surprising, however, is that the benefit was so immediate.
“Our research suggests that encouraging adolescents to spend more time exercising during the day may help their sleep health later that night,” said Lindsay Master, the study’s lead author and a data scientist at Penn State, in a released statement.
How the study was done
For the study, Master and her colleagues analyzed data collected from 417 teenagers who are part of the Fragile Families and Child Wellbeing study, which has been following a large group of children born — mostly to unmarried parents — between 1998 and 2000 in 20 U.S. cities.
When the teens were 15 years old, they wore an accelerometer on their wrists and hips for a week. The device measured their sleep and physical activity.
On average, the teens fell asleep at 12:04 a.m. and woke up at 7:49 a.m., for an average sleep time of 7.7 hours. During the day, they spent an average of 45 minutes doing some kind of moderate to vigorous physical activity and spent roughly 6.5 hours being sedentary (in other words, sitting or lying down).
On the days when the teens increased their physical activity, however, they tended to have a better night’s sleep. Every extra hour of physical activity was associated with an 18-minute earlier bedtime and with 10 more minutes of sleep throughout the entire night, on average. Increased physical activity was also linked to less waking up during the night.
Limitations and implications
The study is observational, so it can’t prove a connection between exercise (or lack of it) and sleep duration and quality. Also, the teens were followed for only one week out of the year. Both sleep and physical activity patterns are known to vary with the seasons — factors that may have affected this study’s findings.
But the study has its strengths, too. “One of the strengths of this study was using the devices to get precise measurements about sleep and activity instead of asking participants about their own behavior, which can sometimes be skewed,” said Master. “The hip device measured activity during the day, and the wrist measured what time the participants fell asleep and woke up, and also how efficiently they slept, which means how often they were sleeping versus tossing and turning.”
Increasing physical activity isn’t the only answer to concerns about what Master and her colleagues call the “alarming rate of sleep insufficiency among adolescents.” Delaying school start times, which addresses the now widely-accepted fact that the internal biological clocks of adolescents causes them to naturally stay up late, is also important.
But increasing physical activity should be encouraged, too — and for people of all ages.
“Becoming our best selves means being more like our best selves more often,” said Orfeu Buxton, the study’s senior author and a professor of biobehavioral health at Penn State, in a released statement. “We were able to show that the beneficial effects of exercise and sleep go together, and that health risk behaviors like sedentary time affect sleep that same night.”
“So if we can encourage people to engage in more physical activity and better sleep health behaviors on a more regular basis, it could improve their health over time,” he added.