As childhood obesity reaches epidemic proportions, parents who are concerned about their children's weight might want to encourage an early-to-bed policy. A new study finds that among the primary-school set, losing sleep is linked with gaining pounds.

Earlier studies had correlated weight with sleep patterns but inevitably raised the question of "which came first, the chicken or the egg," explains Julie C. Lumeng of the University of Michigan in Ann Arbor, who led the new study.

Her team questioned the parents of 785 children from all over the United States about sleep patterns when their kids were in third grade. Three years later, they questioned the parents again.

By sixth grade, 18 percent of the children had become obese. "I expected we'd find that this [sleep link with obesity] was just a bunch of bunk," says Lumeng, a pediatrician. "But the relationship proved so robust that no matter how we looked at it, we couldn't make it go away." In her team's analysis, obesity didn't track with children's behavior, gender, or race, nor did it correlate with the strictness or laxity of parental discipline.

Although the average sleep time among all children was 9.5 hours per day throughout the 3 years analyzed, individual sleep times varied widely. In sixth grade, each additional hour above the average sleep time correlated with a 20 percent lower chance of obesity. Sleep patterns were even more strongly linked when the children were in third grade, when every extra hour of sleep was associated with a 40 percent drop in a child's chance of becoming obese by sixth grade. The researchers report their findings in the November Pediatrics.

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