US scientists have found that not enough sleep probably leads to children becoming overweight.

The findings are published in the journal Child Development.

"Our study suggests that earlier bedtimes, later wake times and later school start times could be an important and relatively low-cost strategy to help reduce childhood weight problems," said Emily Snell, lead author and doctoral student in human development and social policy at Northwestern University, Evanston, Illinois.

Previous studies have linked lack of sleep in children to other factors such as emotional and social wellbeing and school performance but this longitudinal study is the first to find a a likely causal link to becoming overweight. It separates the tangled questions of whether overweight children are poor sleepers or whether lack of sleep leads to being overweight and addresses the latter.

Emily Snell wrote the article with her two co-researchers, Emma Adam and Greg Duncan, assistant professor and professor of education and social policy, at the University's Institute for Policy Research and Center for Social Disparities and Health.

Snell and her colleagues examined nationwide representative samples of data from 2,182 children aged 3 to 18, in two sets taken 5 years apart to look at relationships between sleep, Body Mass Index (BMI) and overweight status.

The data came from the Child Development Supplement of the Panel Survey of Income Dynamics and is based on time diaries kept by parents and care givers, and for the older age group, by the children themselves. The diaries record events such as time of going to bed, hours asleep, and time of waking up for a weekday and weekend day.

"We found even an hour of sleep makes a big difference in weight status," said Snell. She said that an extra hour of sleep reduced the risk of being overweight from 36 to 30 per cent for young children and from 34 to 30 per cent for older children. The results are independent of gender, race, ethnicity and income. And there was no evidence linking height gain to more sleep.

Snell and her colleagues found that for younger children, aged 3 to 8, later bedtimes are more strongly linked to being overweight, whereas in older children, aged 8 to 13, it is earlier wake times that are strongly linked to overweight status.

They also discovered that by the age of 7, the younger children were on average sleeping less than 10 hours a night during the week. For the older children, by the age of 14, the average night's sleep fell to 8.5 hours during the week, and 16 per cent of 13 to 18 year olds were getting less than 7 hours of sleep on a weekday night.

Snell and her team said that many American youngsters are simply not getting enough sleep, and this matter concerns parents, policy makers and health professionals. They suggest that strategies to help children sleep more could make a significant difference to reducing the risk of them becoming overweight.

The National Sleep Foundation say that children between 5 and 12 years old should get 10 to 11 hours sleep a night, and for adolescents this should be no less than 9 hours.