Here's another reason to get the kids to bed early: More sleep may lower their risk of becoming obese. 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 University of Michigan, who led the research.

If there was a magic number for the third-graders, it was nine hours, 45 minutes of sleep. Sleeping more than that lowered the risk significantly.

The study gives parents one more reason to enforce bedtimes, restrict caffeine and yank the TV from the bedroom. The study appears in the November issue of the journal Pediatrics.

Lack of sleep plays havoc with two hormones that are the "yin and yang of appetite regulation," said endocrinologist Eve Van Cauter of the University of Chicago, who was not involved in the new study.

In experiments by Van Cauter and others, sleep-deprived adults produced 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 on the couch and eat cookies, Lumeng said.

Dr. Stephen Sheldon, director of sleep medicine at Chicago's Children's Memorial Hospital, praised the study and called for more research. He said children's sleep may be disturbed by breathing problems — some caused by overweight, such as sleep apnea, and some caused by enlarged tonsils and adenoids.

"I'm not so sure we have enough information yet on cause and effect," said Sheldon, who was not involved in the study.

Researchers used data from an existing federal study and focused on 785 children with complete information on sleep, and height and weight in the third grade and sixth grade. The children lived in 10 U.S. cities.

Mothers were asked: "How much sleep does your child get each day (including naps)?" On average, the third-graders got about 9 1/2 hours sleep, but some slept as little as seven hours and others as much as 12 hours.

Of the children who slept 10 to 12 hours a day, about 12 percent were obese by sixth grade. Many more — 22 percent — were obese in sixth grade of those who slept less than nine hours a day.

The researchers took into account other risk factors for obesity, such as the children's body mass index in third grade, and still found the link between less sleep in third grade and obesity in sixth grade. They acknowledged that factors they did not account for, such as parents' weight or behavior, may have contributed to the risk.

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."