Children who do not take any exercise during the day take longer to fall asleep, research has shown.
It takes an extra three minutes for them to get to sleep for every hour a child is inactive, the research team from Australia and New Zealand said.
Experts from Monash University in Melbourne and the University of Auckland looked at the factors affecting sleep habits and found that exercise has an important role.
The research, which involved 519 children born in 1996 and 1997 and followed them up when they were seven years old, has been published in the journal Archives of Disease in Childhood.
Most children fell asleep within 40 minutes, with an average of 26 minutes, but some spent almost three hours tossing and turning, the team found.
The children wore activity monitors to determine which had sedentary lifestyles and which who took moderate or vigorous exercise.
The children who took part in the more vigorous exercise fell asleep more quickly but those with sedentary lifestyles took longer to fall asleep. Those who fell asleep quickest also tended to sleep for longer.
The researchers said: "As short sleep duration is associated with obesity and lower cognitive performance, community emphasis on the importance of promoting healthy sleep in children is vitally important."