A lack of shuteye over the weekend could be piling extra weight on American children, a sixth of whom are already obese, researchers said Monday.

After logging the nocturnal habits of more than 300 kids 4 to 10 years of age for a week, they found obese kids slept fewer hours and had a more irregular sleep pattern than their slimmer peers.

The researchers acknowledge that the study was not designed to prove that less weekend slumber packed on the pounds, but they say other animal and human studies show sleep can also influence weight.

"We think that the direction of the arrow is you sleep less, you eat more, you exercise less because you're tired, and therefore you gain more weight," said Dr. David Gozal, an expert in childhood sleep problems at the University of Chicago, who led the new work.

For instance, he added, depriving healthy adults of rest will make their bodies act as if they had diabetes, showing signs like a lower tolerance of high blood sugar.

"Over the last 50 years we have seen an increase in obesity rates also for children, and in parallel there have been decreases in the amount of sleep that children get," Gozal said.

To see if the two phenomena are linked, the researchers had children wear a small device that measured their sleep at night.

While kids on average got about 8 hours a night regardless of weight, those who were obese got some 20 minutes less on weekends, and it wasn't as regular as among normal-weight kids.

That's still less than they should be getting, according to the National Heart, Lung and Blood Institute, which recommends at least 9 hours for school-age children and adolescents.

Still, it appeared that repaying the "sleep debt" over the weekend made a difference, said Gozal.

"If you slept little during the week but consistently caught up on it over the weekend, then you reduced your risk of obesity from 4-fold to 2.8-fold," he told Reuters Health.

It's not entirely clear how sack time would influence weight, but hormones such as ghrelin and leptin that help regulate appetite are probably involved, researchers say.

Bruce S. McEwen, a biologist at the Rockefeller University in New York who studies hormones and behavior, said the new findings made sense.

In a recent study, he found lab mice forced to live on a short light-dark schedule became obese in just 1 month, and also showed evidence of declining cognitive function.

"There is likely to be a causal chain reaction," he told Reuters Health in an e-mail.

Gozal said his findings should be a wake-up call for both parents and politicians.

"Our society thinks of sleep as a commodity that can be sacrificed easily," he said. "We look at people that sleep less as if they were heroes. Better education of parents and children about getting regular sleep, and not sacrificing it for TV, etc., would lead to a healthier society."