Nov. 5 (HealthDay News) -- While the connection between a child's weight and the amount of sleep that child gets may not be immediately apparent, new research has found a strong correlation between the two.

Sixth-graders who averaged less than 8.5 hours of sleep a night had a 23 percent rate of obesity, while their well-rested peers who averaged more than 9.25 hours of sleep had an obesity rate of just 12 percent, according to a new study.

"We found that children who got less sleep were more likely to be obese," said the study's lead author, Dr. Julie Lumeng, an assistant research scientist at the University of Michigan Center for Human Growth and Development.

Lumeng said that even after compensating for other factors, such as the home environment, the link between less sleep and heavier weight was still apparent.

The study results are published in the November issue of the journal Pediatrics.

Lumeng said there are three likely reasons why sleep might affect weight. First, if children don't get enough sleep at night, they'll be less likely to run around and get exercise during the day. Second, when kids are tired, they're more irritable and may reach for junk food to help regulate their mood. And, finally, what Lumeng called a "hot area for future research" is the possible connection between sleep and fat metabolism. She said there have been studies done with adults that have shown that a lack of sleep may disrupt the secretion of hormones involved in appetite and metabolism, such as leptin and insulin.

The new study included 785 children who were in third grade at the start of the trial. Most were white -- 81 percent -- and half were female.

Parents were interviewed about their children's sleep habits when the youngsters were in third grade and then again when they were in sixth grade. The researchers also measured height and weight. Obesity was defined as having a body mass index (BMI, a ratio of weight to height) higher than the 95th percentile for age and gender, according to Lumeng. Eighteen percent of the children were obese in sixth grade.

The researchers also took into account maternal education, race, the quality of the home environment and parenting skills to see if those factors affected a child's weight.

No matter what a child weighed in third grade, too little sleep correlated with being obese in sixth grade. And, short sleep duration in sixth grade also correlated with excess weight in sixth grade, according to the study.

Third-graders who got less than nine hours and 45 minutes of sleep a night had an obesity prevalence of about 20 percent, while those who got more than nine hours and 45 minutes of sleep had obesity rates of about 12 percent, Lumeng said.

Those who were short-changing sleep in third grade had 40 percent higher odds of being obese in sixth grade, and sixth-graders who weren't getting enough sleep were 20 percent more likely to be obese, compared to their well-rested counterparts.

Lumeng said the researchers weren't able to find a statistical association between quality of sleep and obesity. But, she said that without a lab-based sleep study, it's difficult to objectively assess the quality of sleep, so there may be an association that this study wasn't able to uncover.

Dr. Stephen Sheldon, director of the Sleep Medicine Center at Children's Memorial Hospital in Chicago, said he would've liked to see sleep studies so the researchers could have known more about the quality of sleep these children were getting, such as how much REM sleep did they get and how fragmented was the sleep?

But, he said, the bottom line is that "pediatricians and parents really need to start paying closer attention to sleep-wake habits. In this society, we put a premium on being awake, and that premium may hurt us in the long run. Sleep may be as important as food to our health and well-being," said Sheldon, who's also a professor of pediatrics at the Northwestern University Feinberg School of Medicine.

Both Lumeng and Sheldon recommended trying to keep a consistent sleep schedule. Bedtimes and wake times are both important -- for children and adults. Sheldon said it's usually OK to vary your sleep times a little bit on the weekend, about an hour or so, but, he cautioned, "Letting you child sleep till noon or mid-afternoon is inviting trouble."

Lumeng also recommended that children not have a TV in their bedroom, because it can make it more difficult to fall asleep.

SOURCES: Julie Lumeng, M.D., assistant research scientist, University of Michigan Center for Human Growth and Development, and assistant professor, department of pediatrics and communicable diseases, C.S. Mott Children's Hospital, Ann Arbor, Mich.; Stephen Sheldon, M.D., director, Sleep Medicine Center, Children's Memorial Hospital, and professor of pediatrics, Northwestern University Feinberg School of Medicine, Chicago; November 2007, Pediatrics