While night owls may crow about their high IQs, heightened creativity and supposed “night strength,” there is at least one disadvantage to the late life: Night owls are more likely to gain weight as they age, according to a new study by researchers at the University of California, Berkeley.
Researcher Lauren Asarnow found that each additional hour participants put off sleep was linked to an increase of 2.1 points on the body mass index scale over a five-year period. Sleeping the same number of hours as early risers didn’t seem to change this relationship, and neither did exercise or the amount of time they spent in front of screens.
“These results highlight adolescent bedtimes, not just total sleep time, as a potential target for weight management during the transition to adulthood,” said Asarnow in a statement.
Sleep scientists already knew that teens who don’t get enough sleep are more likely to be overweight. But Asarnow’s research suggests it’s not just about the length of sleep; it’s about the time they fall asleep, too.
In her study, a sample of 3,342 nationally representative teen participants answered questions about their sleep habits and lifestyle. The study started with a first wave of teens in 1994 and continued through 2009, when the last group hit adulthood.
Those who went to sleep later were more likely to increase their BMI (proportion of weight to height) over time, which leads Asarnow to suspect that teens who sleep earlier will “set their weight on a healthier course as they emerge into adulthood.”
There are a few caveats for the findings, above all that teens self-reported their sleep time and duration in a questionnaire, as opposed to using the gold standard of sleep measurements: sleep diaries, motion sensors and “forced desynchrony,” a process in which researchers re-set a person’s circadian rhythms by exposing them to a certain lightness/darkness schedule. Such methods are impractical for large population studies like hers, Asarnow writes, but they should continue to be used in smaller studies to get more accurate measurements.
In addition to the increased BMI, the study also found that the link between sleeping late and weight gain was even stronger if the participant also ate fast food, which suggests that if teens cut back on eating junk, they may not be at as high a risk.
“This does suggest that if you eat fast food and you go to bed late you are even more at risk for weight gain over time,”Asarnow told HuffPost. “Indeed, fast food consumption might be another target for weight gain prevention.”
The National Sleep Foundation recommends that teens sleep between eight to ten hours every night, but only about 15 percent of teens report sleeping at least 8.5 hours on weeknights. They need more rest than adults because their bodies are rapidly growing. However, because of the hormonal changes taking place in their body, teens will feel sleepy two hours later than they did as children, pushing their natural bedtimes to 11 p.m. Unfortunately, most high schools in the U.S. start at around 8 a.m., which explains why teens report getting an average of only seven hours of sleep a night.
While Asarnow’s research doesn’t explain why there’s a link between late bed times and weight gain over time, there are a number of possible explanations. Sleep helps people maintain healthy weights because rest helps regulate the hormones that control our appetites. Getting more sleep also means you’re energized throughout the day, which ups the chances for exercise. Finally, more sleep can also aid in healthy weight maintenance simply because the hours you spend sleeping are hours you aren’t eating.
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