Children who do not get enough sleep are more likely to be overweight than those who get more, according to a study published on Wednesday that tracked more than 2,000 U.S. kids for five years.

Researchers at Northwestern University in Evanston, Illinois, used detailed diaries kept by families to examine children's sleep behavior and its relationship with weight.

"Children who get less sleep tend to weigh more five years later," lead researcher Emily Snell said in an interview.

Snell and colleagues Emma Adam and Greg Duncan determined that an extra hour of sleep cut the likelihood of being overweight from 36 percent to 30 percent in children ages 3 to 8, and from 34 to 30 percent in those ages 8 to 13.

The study, published in the journal Child Development, involved 2,281 children taking part in a nationally representative survey. They were ages 3 to 12 at the start of the study in 1997. Follow-up data was collected five years later.

The diaries recorded the number of hours the kids slept, what time they went to bed and what time they woke up. Their height and weight also were recorded.

The researchers found that children who got less sleep were more likely to be overweight and have higher body mass index measures than those who got more sleep, even when factors such as race, ethnicity and parents' income and educational level were considered.

POSSIBLE EXPLANATIONS

The study noted that sleep experts recommend that children ages 5 to 12 sleep for 10 to 11 hours a night and adolescents sleep for 8 to 9 hours. But the researchers said children in the study at age 7 on average got less than 10 hours of sleep on weekdays and at age 14 got 8.5 hours of sleep on weekdays.

The study did not examine why children who slept less tended to weigh more, but Snell cited a few possible explanations.

Not getting enough sleep may affect hormones that influence appetite, Snell said. Getting less sleep -- for example, staying up an hour later at night -- may provide more opportunity to eat, she added. And she said not getting enough sleep may leave a person more lethargic, cutting down on exercise.

Snell said on weekdays, school schedules can dictate when children must wake up, but parents can control bedtime.

"Particularly for younger children who need 10 to 11 hours of sleep at night, if their wake-up time has to be 6:30 or 7 (a.m.) for school, we encourage parents to sort of aim for the 8 o'clock hour for bedtime," Snell said.

The researchers noted there is growing evidence linking sleep to children's cognitive and social functioning, with previous research connecting sleep problems and too little sleep to maladjustment in preschoolers and depression and school problems in adolescents.