A new study finds that teens who went to bed early and got up early were thinner and fitter than kids who slept late. The findings, published in the journal Sleep, suggest that it’s not just the amount of sleep that kids get, but when they get it that matters.
The researchers tracked the sleeping habits, weight and use of free time of 2,200 Australian children aged 9 to 16 over four days. They found that all the kids slept about the same amount of time nightly, but that kids who went to bed and awoke later were 1.5 times more likely to be obese and twice as likely to have a sedentary lifestyle, compared with the early birds.
Lifestyle may be a key contributor to the weight difference, the study found: kids who stayed up late into the night tended to use that time doing sedentary things. They were nearly three times more likely to spend more time watching TV, using the computer or playing video games than their early-rising peers.
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Overall, night owls replaced about 30 minutes of physical activity a day with sedentary behaviors, compared with the early birds. As previous research has suggested, couch-potato pastimes like watching TV conspire to make you fat because they come along with so many other unhealthy influences, such as eating junky snacks and watching fast-food ads.
The study found further that early birds went to bed 70 to 90 minutes earlier than late-sleepers and woke up 60 to 80 minutes earlier. They got nearly a half-hour more exercise than kids who stayed up late. Late-sleeping children also tended to come from lower-income families, live in cities and have part-time jobs; they had few siblings.
Teens are already naturally wired to stay up late and sleep late in the morning, but the study authors suggest that parents keep an eye out for extreme sleeping behavior.
Meredith Melnick is a reporter at TIME. Find her on Twitter at @MeredithCM. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.