July 22, 2009 -- As parents watch their children swimming or playing tag, they often comment happily to each other about how well their kids will sleep that night. Now there's scientific data to back up that playground chatter.

A new study shows that active kids fall asleep faster. The study also shows that the longer it takes to fall asleep, the shorter the total sleep time. This is important because bad sleep patterns in children have been associated with poorer school performance and an increased risk of being obese or overweight.

The study, published in Archives of Disease in Childhood, included 871 children of European-descent mothers recruited at birth. Seven years later, 591 of the children participated in the current study. Participants were part of a broader study called the Auckland Birthweight Collaborative Study.

For the sleep research, participants' sleep problems and behavior patterns were assessed through questionnaires. Also, participants wore activity monitors around their waists for 24 hours to measure sleep and physical activity level.

Median sleep latency -- or how long it took a child to fall asleep -- was 26 minutes. The higher the child's activity rate, the shorter the sleep latency tended to be.

For every daytime hour the child was sedentary, he or she spent an additional 3.1 minutes trying to fall asleep.

Shorter sleep latency also was linked to longer sleep duration.

"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 write.