Children under 12 who sleep more at night tend to weigh less in later years compared with those who don't get as many winks, researchers in the U.S. say.

Researchers at Northwestern University in Illinois looked at more than 2,200 children aged three to 12 from across the country. The children were followed for five years.

"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 the study's lead researcher, Emily Snell of the School of Education and Social Policy and the Institute for Policy Research at Northwestern.

Diaries were used to record the total number of hours slept, when the children went to bed and the number of times they woke up. Heights and weights were also measured.

Children who slept nine hours instead of 10 were about one-fifth as likely to be overweight five years later, the researchers report in the January/February issue of the journal Child Development.

Getting an extra hour of sleep seemed to cut the likelihood of being overweight to 30 per cent from 36 per cent in children aged 3 to 8, and to 30 per cent from 34 per cent in those aged 8 to 13, after taking into account other factors that influence weight, such as race, ethnicity and parents' income and education levels.

Sleep experts recommend that children aged five to 12get 10 to 11 hours a night of sleep, while teens should hit the sack for eight to nine hours.

In reality, the children in the study got less than 10 hours of sleep on weekdays at age seven, and 8.5 hours of sleep on weekdays at age 14.

It's thought that lack of sleep may affect hormones that influence appetite. Staying up later may also provide more chances to snack while leaving children feeling too lethargic to exercise.

Lack of sleep in children has also been linked to poor cognitive and social functioning.