Getting too little sleep increases a young child's risk for becoming overweight later in life, a small new study suggests.

Examining a group of children in New Zealand over several years, researchers found that the 3-, 4- and 5-year-olds who slept the least were at an increased risk for a high body mass index , or BMI, at age 7. BMI is a ratio of height and weight and is considered an indictor of body fatness.

The researchers said they aren't sure what's behind the link, but that it could have to do with the amount of time kids are awake and thus can be eating. Or perhaps it's a physiological response the body has to fatigue, they said.

With two different measures of children's body-fat percentage, the researchers found the differences in BMI were due to differences in fat mass rather than muscle tissue. The results held even after the researchers accounted for other factors that might affect body mass, such as physical activity and diet.

The findings could be important in light of the epidemic of childhood obesity . The number of obese children and adolescents in the United States has tripled over the last generation and now stands at 17 percent of the population.

The obesity boom has coincided with a general reduction in the amount of sleep kids get , the researchers say.

During toddler years, children average about 12 hours of sleep a day, though some need more than others, said study researcher Barry Taylor, a professor and pediatrician at the University of Otago in Dunedin, New Zealand.

Sleep loss and weight gain

Studies have consistently shown a link between shorter sleeping time and an increased risk of obesity in children, teens and adults. However, most studies in children have relied on parents' reports of their kids' sleep patterns, rather than on an objective measure of snooze time. In addition, it was unclear whether the later weight gain was due to an increase in fat or non-fat tissue.

The study involved 244 New Zealand children, who were followed from age 3 until they were 7. Every six months, the kids had their height, weight, BMI and body fat measured. To get a measure of their sleep patterns, the researchers had children wear belts that detect movement, for five days. They also answered dietary questionnaires accounting for a three-day period.

On average, the children slept 11 hours a day when they were 3, 4 and 5. Almost all the children in the study got between 10 and 12 hours of sleep per night.

Every  additional hour of sleep over the range of sleep values recorded, was associated with a 61 percent reduction in the risk of being overweight or obese at age 7. Each extra hour of sleep during the younger ages (3- 5) was also associated with a reduction in BMI of 0.49, or about 1.5 pounds, at age 7.

The study extends what we know about sleep loss and obesity risk in children, said Chantelle Hart, an assistant professor of psychiatry and human behavior at Brown University School of Medicine. However, it can't tell us exactly how many hours of sleep are needed for the protective effect, added Hart, who was not involved in the study.

"All we can say is kids who slept more had a reduced risk of obesity," Hart said. "We can't talk about thresholds."

How does it happen?

Researchers aren't sure exactly why sleep loss might cause weight gain, but they have a few ideas.

First, if you sleep less, you're awake more and have more of an opportunity to eat (though you also have more active time). Also, sleep loss may lead to fatigue and a decrease in energy expenditure, since a lethargic person is less likely to move about. There is also some evidence that sleep loss can cause an increase in ghrelin, the hormone that stimulates appetite. Sleep loss may also influence the way the body expends energy to heat itself, a process called thermoregulation.

More research is needed to verify the results, Taylor said. Studies also should look into whether increasing children's sleep hours changes how they put on weight, Taylor said.

The study was published online May 26 in the British Medical Journal.

Pass it on: Young children who don't get enough sleep may be at an increased risk for becoming overweight or obese.

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