Preschoolers who do not get enough sleep are at higher risk of becoming overweight, researchers in New Zealand have concluded.

Getting less sleep seems to be related to increased body weight in children but doctors aren't sure how. 

To learn more, Prof. Rachael Taylor of the human nutrition department at the University of Otago in Dunedin and her colleagues  followed 244 children between the ages of three and seven. 

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Young children who don't get enough sleep risk becoming overweight. (iStock)

The children had their weight, height, body mass index and body composition measured every six months. Parents filled in questionnaires about the sleep and diet habits of their sons and daughters at three, four and five years of age. At those ages, the children also wore accelerometers to monitor their movements. 

The boys and girls averaged 11 hours of sleep per day at all three ages.

Each extra hour of sleep per night at age three to five was associated with a reduction in BMI of 0.49 times and a 0.61 times reduction in the risk of being overweight or obese at age seven, the team found.

The researchers said the differences in BMI were from fat mass, which points to how poor sleep harms body composition in children.

The link remained after taking factors such as gender and physical activity into account.

As for why, the researchers proposed that having more time to eat and changes in hormones in the brain could affect appetite.

"The study is a valuable contribution to the understanding of the causal pathway whereby reduced sleep duration may directly contribute to overweight and obesity in children," Professor Francesco Cappuccio and Associate Professor Michelle Miller from the University of Warwick said in a journal editorial accompanying the study.

Strengths of the study included use of accelerometers and sleep diaries, which offered reliable, direct and repeated measurements of time in bed and time asleep, the editorial said.

"It would do no harm to advise people that a sustained curtailment of sleeping time might contribute to long-term ill-health in adults and children," the pair concluded.

A drawback is that the response rate in the questionnaires was only 59 per cent.

The Canadian Pediatric Society advises that children aged three to five should get 10 to 12 hours of sleep.

The study was funded by the University of Otago, the Child Health Research Foundation, the New Zealand Heart Foundation, and the Dean’s Bequest-AAW Jones Trust.