WEDNESDAY, Feb. 7 (HealthDay News) -- Eating right and being active are well-known ways to try to keep a child's weight in check, but a new study points to another potential weapon in the fight against childhood obesity -- sleep.

Researchers at Northwestern University found that children who get more sleep tend to have a lower body mass index (BMI, a ratio of weight to height) and are less likely to be overweight five years later, than kids who get less sleep.

Sleeping an extra hour a night reduced the chance of being overweight from 36 percent to 30 percent in young children, and from 34 percent to 30 percent in older children.

The study is published in the January/February issue of the journal Child Development.

"Our study adds to the growing literature about the connection between sleep and weight," said Emily Snell, the study's lead author and a doctoral student in Northwestern's department of human development and social policy. "Other studies have found that sleep and weight are related in adults and kids, but it's not clear if sleep affects weight, or vice versa. We accounted for that by factoring in how much the kids already weighed," Snell said.

The researchers did this by collecting data in two waves. They started by looking at 2,281 children from a national survey called the Child Development Supplement of the Panel Survey of Income Dynamics. The kids were ages of 3 to 12 at the start of the study, and follow-up information was collected five years later. Parents and/or the children were given "time diaries" to record sleep information, such as the total number of hours the child slept, bedtimes, and wake times.

Analysis of the diaries showed that children who slept less, went to bed later, or got up earlier during the first assessment had higher BMIs five years later and were more likely to be overweight. The research also showed that a later bedtime had a greater effect on whether children aged 3 to 8 became overweight, while earlier wake times played a greater role for children aged 8 to 13.

"The effect of sleep on the weight of younger kids came through their bedtime. An earlier bedtime seemed to matter more, and bedtime is a place where parents have control," Snell said. "For older kids, wake time mattered more. I'm not sure how comfortable I am telling school districts that if they start school early their kids will end up fat. But it does add to the evidence that a very early start time (7 a.m. or earlier), especially for pre-adolescents and teens, is not in line with adolescents biological clocks."

Snell added that the study also found a discouraging trend in how much -- or little -- sleep kids are getting, particularly on weeknights. By age 7, children were sleeping, on average, less than 10 hours on weekdays, and this dropped to 8.5 hours by age 14. Sixteen percent of adolescents aged 13 to 18 slept less than seven hours on weeknights.

The National Sleep Foundation recommends that 5- to 12-year-olds get 10 to 11 hours of sleep nightly and that teens get eight to nine hours.

Lack of sleep not only affects a child's weight, but it can have other ramifications, said Dr. Shahrad Taheri, a lecturer in medicine and endocrinology at the University of Bristol, in Great Britain.

"We are increasingly understanding that sleep has multiple functions," he said. "Short sleep has been associated with poor educational performance, alcohol use and addiction, poor immune responses, hypertension, diabetes, and cardiovascular disease."

Researchers don't know why sleep and childhood obesity may be related, but Snell said there are many potential pathways that link the two. For instance, not getting enough sleep can leave a child tired, and therefore less likely to be active. Also, being awake longer provides more time for eating.

Lack of sleep may also disrupt hormones that influence metabolism and hunger.

"Literature suggests that with restricted sleep comes changes in certain hormones that could alter intake of food," said Dr. Robert Vorona, a sleep specialist and an associate professor at Eastern Virginia Medical School in Norfolk, Va. "Leptin (which decreases food intake) and ghrelin (which increases food intake) are respectively reduced and increased by lack of sleep."

But, researchers don't recommend that parents wait to fully understand what links sleep and obesity before taking action. "If our kids got better sleep, then maybe we can make a dent in the obesity statistics," Taheri said. "You only need a 100 calorie surplus a day to put on significant weight over time, so every little effort helps."

More information

For more on children and sleep, visit the National Sleep Foundation.