Weighing too much can damage your health, and obesity is a growing problem for both kids and adults around the world.

Sleep might be one answer to the problem. A new study has found that elementary school students who slept too little were more likely to gain pounds.

In the United States today, some 9 million children over the age of 6 are obese.

Past studies have shown a link between sleeping less and weighing more, but scientists have had a tough time determining "which came first, the chicken or the egg," says Julie C. Lumeng of the University of Michigan in Ann Arbor.

In other words, it hasn't been clear whether kids who weigh too much have trouble sleeping, or whether sleeping less leads to weight gain. Both scenarios seemed equally possible.

To get a better idea of which causes which, Lumeng and colleagues interviewed the parents of 785 third graders from around the United States. The parents answered questions about how well their kids slept that year. Three years later, the parents answered the same questions.

By sixth grade, 18 percent of kids involved in the study were obese. The scientists found no relationship between weight and the students' race or gender. It also didn't matter how strict their parents were, or whether they were boys or girls. Obesity struck all of these groups equally.

Instead, sleep seemed to be the key factor. Over the 3 years of the study, the children averaged a healthy 9.5 hours of sleep a night. Some kids, however, slept a lot more--or less--than others.

For the sixth graders, every hour of sleep above the 9.5-hour average was linked to a 20 percent lower risk of being obese. Sleep appeared doubly important for the third graders. Every extra hour of sleep they got was linked to a 40 percent drop in obesity by sixth grade.

"I expected we'd find that this [sleep link with obesity] was just a bunch of bunk," says Lumeng, a pediatrician. But their findings were convincing. No matter how her team looked at the link, "we couldn't make it go away."—Emily Sohn