But for sedentary children, every hour of inactivity increased the time it took to fall asleep by three minutes, Mitchell's group found.
About 16 percent of parents of school-age children say their child has difficulty falling asleep, according to the researchers, who noted that poor sleep patterns have been linked to poorer school performance and an increased risk of a child becoming overweight or obese.
"This study emphasizes the importance of physical activity for children, not only for fitness, cardiovascular health and weight control, but also for sleep," the researchers concluded.
Taking longer to fall asleep, though, is not really a problem, said Dr. David Rapoport, director of the New York University Sleep Disorders Center.
"The finding goes with our biases," Rapoport said. "The more you run around as a child, the better you will sleep at night. But it suggests that if you don't do anything all day, you may need less sleep."
"It is not clear that if you don't do the exercise and don't get to sleep, or get less sleep, [that] is itself a bad thing," Rapoport said.
Is less sleep, then, something to worry about?
"Far from it," he said. "I see this as something which we were designed by nature to do. The purpose of sleep is to recover from activity, and what this is showing is that that link is quite tight in the child. If the child exercises, they need more sleep and they get it more easily."
For sedentary children, Rapoport said, the study indicates that they may be going to bed too early, when they are not ready to sleep.
"I don't know if there is a problem, but there is a simple cure," he said. "See to it that the kid gets enough exercise during the day because then they will fall asleep more rapidly."
All rights reserved