A lack of shuteye over the weekend could be piling extra weight onto US children, a sixth of whom are already obese, a study says.

The research, published in Pediatrics, followed the sleeping habits of 300 children between 4 to 10 years of age for a week and found that obese children slept fewer hours, and had more irregular sleep patterns, than their slimmer peers.

"We think 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 David Gozal from the Corner Children's Hospital and University of Chicago, who led the study.

"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 and his team did acknowledge their study wasn't designed to prove that less weekend slumber packed on the weight, but noted that other animal and human studies show sleep can also influence weight.

To check for links between increased weight and decreased sleep, researchers had the children wear a small device that measured their sleep at night.

While children on average got about 8 hours of sleep 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 children.

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

Repaying the "sleep debt" over the weekend did appear to help, Gozal said.

"If you sleep little during the week but consistently caught up on it over the weekend, then you reduced your risk of obesity from fourfold to twofold," he said.

Research has not pinpointed how sleep time would influence weight, but hormones that regulate appetite, such as ghrelin and leptin, are probably involved.

Biologist Bruce McEwen, who studies hormones and behavior at Rockefeller University in New York and was not involved in the study, said the findings made sense.

In a recent study, he found that lab mice forced to live on a short light-dark schedule became obese in just one month.

"There is likely to be a causal chain reaction," he said.

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. We look at people that sleep less as if they were heroes," he said.

"Better education of parents and children about getting regular sleep, and not sacrificing it for TV etc, would lead to a healthier society."