Children under the age of five who don't get enough sleep at night are more likely than kids who do get their 40 winks to become obese at a young age, a study published Monday showed.
"We found a robust longitudinal association between duration of nighttime sleep in early life and subsequent obesity measured at five to nine years," wrote the authors of the study in the Archives of Pediatric and Adolescent Medicine, a journal of the American Medical Association.
Researchers led by Janice Bell of the University of Washington, Seattle and Frederick Zimmerman of the University of California, Los Angeles, studied two lots of data -- at baseline and five years later -- for 1,930 children in the United States.
The kids were separated into two groups for the study: ages zero to 59 months, and five to 13 years.
The data analyzed included information known to influence whether a child develops obesity, including parents' weight and the child's physical activity level, as well as how long the children slept at night and whether they napped during the day.
On average, younger children in the study slept 10 hours a night, and older children slept around 9.5 hours, but some children in both age cohorts got as little as five hours' sleep a night.
The data collected five years after baseline showed that 33 percent of the younger cohort and 36 percent of the older cohort of kids were overweight or obese.
"For the younger children, low nighttime sleep at baseline was significantly associated with increased odds of overweight versus normal weight and increased odds of obesity versus overweight at follow-up," the study says.
Nearly one in five US children is obese (17 percent) and more than a third are overweight, the study says, adding that ensuring that very young children get enough sleep at night could play a key role in preventing obesity.
Napping during the daytime, which the younger kids did but the older ones did not, appeared to have no effect on the children's weight, and getting little sleep at night also did not affect the weight of the older children.
"These findings suggest that there is a critical window prior to age five years when nighttime sleep may be important for subsequent obesity status," the study says.
Why sleep affects weight is not precisely known, but the authors of the study said that getting less sleep could lead to "decreased physical activity due to tiredness and increased energy intake" because the waking child has more opportunities to eat.
Another possible reason why lack of sleep leads to weight gain is that the number of hours of shut-eye influences hormones that affect appetite, hunger and metabolism, the study found.