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.