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Researchers have found that people who sleep at least eight hours are more likely to maintain a normal weight than those who sleep less. Now, a new study finds that even for infants and preschoolers, a long night's sleep may be just as important as diet and physical activity. NPR's Patty Neighmond has this report on the study, which appears in the Archives of Pediatrics and Adolescent Medicine.

PATTY NEIGHMOND: Over the last three decades, obesity rates have doubled among children age two to five, and tripled among six- to 11-year-olds. So, Janice Bell wanted to know whether sleep had anything to do with it.

Bell's a researcher at the University of Washington in Seattle. She looked at federal data collected on nearly 2,000 children and compared to those who slept 10 hours or more a night, to those who slept less. She also looked at how much the children weighed over a five-year period. The most striking findings, says Bell, had to do with infants and toddlers. The study appears in the Archives of Pediatrics and Adolescent Medicine.

Ms. JANICE BELL (Researcher, University of Washington in Seattle): They were nearly twice as likely to move from normal weight to overweight; or from overweight to obese, in that five-year period.

NEIGHMOND: Twice as likely to gain weight is a big difference, says Bell, and an important message to parents to help their children get on a routine schedules of a long and solid night's sleep. Especially, she says, because napping during the day did not reduce the risk that these kids would gain weight.

Ms. BELL: We found that their napping didn't have any effect on their later obesity, whereas the nighttime sleep was significant. So, that led us to conclude that napping didn't seem to be a substitute for a nighttime sleep in terms of obesity prevention.

NEIGHMOND: Bell's study didn't examine why it may be that children who sleep less gain weight - but she has some theories.

Ms. BELL: It may be that children who don't sleep enough at night are too tired to engage in the kind of physical activity that may prevent obesity. It could also be that there's a relationships between appetite hormones and later obesity. We know that that relationship between appetite hormones and sleep exists in adults.

NEIGHMOND: And in adults, one sleep expert who's studied the relationship between sleep deprivation and appetite is Dr. Emmanuel Mignot, who directs the Center for Sleep Sciences and Medicine at Stanford University.

Dr. EMMANUEL MIGNOT (Director, Center for Sleep Sciences and Medicine, Stanford University): If you start to sleep less, there are certain hormones, such as leptin or ghrelin that change in your blood in a way that stimulates your appetite.

NEIGHMOND: And makes you crave unhealthy foods.

Mr. MIGNOT: In fact, in one study, they compare snack food versus other type of foods, and they found that when you are sleep deprived, you seem to prefer snack food, as opposed to healthy food.

NEIGHMOND: And one reason naps may not work, says Mignot, is because daytime sleep in qualitatively different.

Mr. MIGNOT: Sleep is better during the night. In fact, body temperature already drops so that, in fact, the sleep that we have at night, even in adults, is always more restorative, deeper, than if you try to sleep during the day.

NEIGHMOND: On average, Mignot says, children up to age five need at least 10 hours of sleep a night. Infants and young toddlers, of course, need even more.

Patty Neighmond, NPR News.

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