A lack of sleep is more likely to lead to overweight children than a poor diet or lethargic lifestyle, a new long-term study has found.
New Zealand researchers monitored a random sample of almost 250 children, regularly tracking weight, diet, body composition, physical activity and sleep patterns from the ages of three to seven.
They took into account birth weight, parent's education, income, ethnicity and if their mother was smoking while pregnant - all factors known to affect a child's weight.
Previous studies have found poor sleep is linked to heavier children, but this is the first time such a thorough and long assessment had been done, researcher Professor Barry Taylor said.
Almost a quarter of the Dunedin children surveyed were overweight by the time they were seven, Prof Taylor said.
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"(But) how active you are actually seems to have no effect on whether or not you're overweight at the age of seven," he told AAP.
"The food that you ate had some effect, but actually the biggest effect was short sleep."
He said the children slept an average of 11 hours each night and those that got any less shut-eye were more likely to be overweight, even if other factors were controlled.
"It's a complicated connection," said Prof Taylor, a pediatrician and academic from the Dunedin School of Medicine.
He said the amount of sleep a person got altered the hormones controlling metabolism and appetite, hence, how much one eats.
"We were surprised by how big a factor (sleep was)," Prof Taylor said.
"I was expecting the ... percentage of food eaten as vegetables and fruit would be more important and that activity levels ... would be more important."
Evidence suggests the amount of sleep both children and adults get has dropped significantly in the past 30 years, Prof Taylor said, blaming a "modern lifestyle".
He said children should generally get about nine to 10 hours of sleep a night, but some will need more.
Trials are now under way to see if teaching families how to deliberately increase their child's sleep can alter growth.
"All we can say at this stage is this looks like something that needs to be done," Prof Taylor said.
The study was published in the British Medical Journal.
