Getting inadequate sleep has negative effects on children's social and emotional well-being and school performance. It also increases their risk of being overweight, a new American study indicates.

The study, conducted by Northwestern University, investigated the relationship between sleep, Body Mass Index (BMI) and overweight status in children aged 3 to 18."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," says Emily Snell.

"We found even an hour of sleep makes a big difference in weight status," said Snell, a Northwestern doctoral student in human development and social policy. "Sleeping an additional hour reduced young children's chances of being overweight from 36 percent to 30 percent, while it reduced older children's risk from 34 percent to 30 percent."

The findings also suggest that later bedtimes play a greater role in the overweight status of children aged from 3 to 8, while earlier wake times play a greater role in children aged 8 to 13. No significant differences in the effect of sleep on weight was found between boys and girls nor was there evidence that children who slept more grew more in height.

The researchers used time diaries, in which the parents or caregivers of young children or children old enough to keep diaries themselves recorded all activities - including bedtime, time asleep and wake time - over the course of a weekday and weekend day. In analyzing the diaries, they found troubling age-related trends in sleep behaviour.

By age 7, children were sleeping on average less than 10 hours on weekdays. By age 14, weekday sleep time fell to 8.5 hours. A full 16 percent of adolescents aged 13 to 18 were found to sleep fewer than seven hours on weekday nights. The National Sleep Foundation recommends children aged 5 to 12 years get 10 to 11 hours of sleep and adolescents get eight to nine hours.

"Parents, policymakers and health care providers all are concerned about the obesity epidemic among children," says Snell. "Our results suggest that something as simple as helping children sleep more at night could reduce their risk of being overweight."

Oxford University neuroscientist Russell Foster said recently that young people perform better later in the day and that school results could improve if classes started later.