Children who are more physically active fall asleep faster and slumber longer. (Reuters)

Every hour a child spends sitting around adds another three minutes to how long it takes them to fall asleep, researchers say.

A team from New Zealand confirmed what many parents have long thought: tiring out a child increases the likelihood that he or she will sleep well.

In Thursday's online issue of the journal Archives of Disease in Childhood, Edwin Mitchell, a professor of child health research at the University of Auckland and his colleagues found that for sedentary children, every hour of inactivity during the day increased the time it took them to nod off by 3.1 minutes.

The researchers collected data on the sleep patterns and daytime activity levels of 591 children, who were all seven years old. Participants wore activity monitors around their waists for 24 hours. It did not seem to matter whether the sedentary time was spent watching TV or reading quietly.

The majority fell asleep within 45 minutes, and the average time it took to doze off was 26 minutes.

The study also suggested that the longer it took to fall asleep, the shorter the total sleep time was. This may be important since poor sleeping patterns in children have been linked to poorer performance in school and an increased risk of carrying extra pounds.

"These findings emphasize the importance of physical activity for children, not only for fitness, cardiovascular health and weight control, but also for promoting good sleep," the researchers concluded.

Less sleep for sedentary

For the one in 10 children who regularly had trouble falling asleep quickly, they took about 15.5 minutes more to doze off.

Children who exercised vigorously during the day, fell asleep faster and slept longer.

But the findings may also suggest that children who aren't active during the day may need less sleep, said Dr. David Rapoport, director of the New York University Sleep Disorders Center.

"The purpose of sleep is to recover from activity, and what this is showing is that that link is quite tight in the child," Rapoport told Health Days News.

"If the child exercises, they need more sleep and they get it more easily."

It could be that sedentary children may go to bed too early, when they're not ready to sleep, he said.