March 29, 2007

Children who sleep less more likely to weigh more

By Wendy Leopold

Research indicates that getting inadequate sleep has negative effects on children’s social and emotional well-being and school performance. Now a Northwestern study finds it also increases their risk of being overweight.

The study — conducted in two waves of data collection approximately five years apart — is the first nationally representative, longitudinal investigation of 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. Snell is co-author of “Sleep and the Body Mass Index and Overweight Status of Children and Adolescents.”

“We found even an hour of sleep makes a big difference in weight status,” said researcher Emily Snell. “Sleeping an additional hour reduced young children’s chance of being overweight from 36 percent to 30 percent, while it reduced older children’s risk from 34 percent to 30 percent.”

The Northwestern study not only differs from most other investigations of the effects of sleep on children’s weight in its five-year approach. It also helps disentangle the issue of whether sleep actually affects weight or whether children who already are overweight are simply poor sleepers. In addition, it takes into account the possible effects of other variables including race, ethnicity and income.

Snell co-wrote the article with Northwestern’s Emma K. Adam and Greg J. Duncan, assistant professor and professor of education and social policy, respectively. Adam and Duncan are fellows at the Institute for Policy Research and Center for Social Disparities and Health. Snell is a doctoral student in the School of Education and Social Policy.

Their 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 behavior.

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.