Letting children sleep late on weekends and holidays might help them avoid becoming overweight or obese, a new study suggests.

Researchers in Hong Kong found that children who got less sleep tended to be heavier (as measured by body mass index, or BMI) than children who slept more. But among children who slept less than eight hours a night, those who compensated for their weekday sleep deficit by sleeping late on weekends or holidays were significantly less likely to be overweight or obese.

The study, which confirmed previous research linking sleep deficits to obesity in children, also found that, on average, children slept significantly longer on weekends and holidays than on school weekdays. However, the overweight children tended to get less weekend-holiday sleep than their normal-weight peers.

The researchers didn’t determine why obese and overweight children were less likely to sleep late on holidays or weekends, but noted that they tended to spend more time doing homework and watching TV than their normal-weight peers.

Biological factors might also play a role in the compressed sleep cycle, they said.

“There’s a lot of evidence linking short sleep duration to higher body mass,” said Kristen Knutson, assistant professor of medicine at the University of Chicago, who was not involved in the study. “What’s unique about this study is that it’s the first to show that extending sleep on weekends may help with avoiding weight gain.”

Still, the researchers urged caution in the interpretation of their findings, acknowledging that “an irregular sleep-wake schedule and insufficient sleep among school-aged children and adolescents has been documented with a variety of serious repercussions, including increased daytime sleepiness, academic difficulties, and mood and behavioral problems.”

The precise nature of the link between short sleep duration and obesity remains unclear, said Mary A. Carskadon, professor of psychiatry and human behaviour at Brown University’s Alpert Medical School in Providence, R.I., and director of chronobiology at Bradley Hospital in East Providence.

“Evidence has shown that there are changes in satiety and in levels of the hunger hormones leptin and ghrelin,” Carskadon said. “But there’s also evidence that kids who are not getting enough sleep get less physical activity, perhaps simply because they’re too tired. It’s just not cut-and-dried.”

The study authors noted that “reduced sleep duration has become a hallmark of modern society, with people generally sleeping one to two hours less than a few decades ago.”

Experts say that adolescents and pre-pubertal children generally do best with 9.5 to 10 hours of sleep a night, younger children a bit more.

The one-year study, led by Yun Kwok Wing of The Chinese University of Hong Kong, used questionnaires to track the sleep habits, lifestyle, height and weight of 5,159 local children aged 5 to 15 years.

The findings, published in the November issue of Pediatrics, could be helpful in preventing and managing childhood obesity, the authors noted.

For now, parents should take note of their children’s wake-sleep cycles in light of other behavioral and environmental factors, the researchers advised.

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