Obesity is a growing problem for both kids and adults around the world as weighing too much can damage your health. Researchers believe that the amount of sleep you get might be a key factor in determining your chances of being obese. A new study has found that elementary school students who slept too little were more likely to gain weight.

Past studies have shown a link between sleeping less and weighing more, but 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, researchers 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 that gaining weight has nothing to do with a student’s race or gender. Sleep, instead, 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. So if you are a weight-watcher it does make sense to spend an extra hour in the bed.