CHICAGO -- Here's another reason to get the kids to bed early: More sleep appears to lower their risk of obesity.
Researchers have found that every additional hour per night a third-grader spends sleeping reduces the child's chances of being obese in sixth grade by 40 percent.
The less sleep they got, the more likely the children were to be obese in sixth grade, no matter what the child's weight was in third grade, said Dr. Julie Lumeng of the Univer-sity of Michigan, who led the research.
If there was a magic sleep number for the third-graders, it was nine hours, 45 minutes. Sleeping more than that cut the risk significantly.
The study appears in the November issue of the journal Pediatrics.
Lack of sleep plays havoc with hormones that are the "yin and yang of appetite regulation," said endocrinologist Eve Van Cauter of the University of Chi-cago, who was not involved in the study. Studies by Van Cauter and others have shown sleep-deprived adults made more ghrelin, a hormone that promotes hunger, and less leptin, a hormone that signals fullness.
Another explanation: Tired kids are less likely to exercise and more likely to sit and eat, Lumeng said.
Jodi Mindell of the Children's Hospital of Philadelphia's Sleep Center noted there are plenty of other reasons for encouraging good sleep habits, such as success in school.
"I don't want parents to think, 'If I get her to sleep, she's not going to be overweight,' " Mindell said. "I think this is a small piece in the picture."