Researchers have said children who do not exercise during the day take longer to fall asleep at night. The study showed that for every hour of inactivity it takes children an extra three minutes to drop off.

The study of 519 seven-year-olds children was undertaken by experts from Monash University in Melbourne, Australia, and the University of Auckland in New Zealand. The researchers noted that although most children fell asleep within 40 minutes, with an average of 26 minutes, some children took almost three hours.

The children wore activity monitors to determine which had sedentary lifestyles and which moderate or vigorous activity. They noted that children who took part in the more vigorous sort of exercise fell asleep faster but those with sedentary lifestyles took longer to fall asleep.

They also noted that children who fell asleep faster also tended to sleep for longer, raising the possibility of this being "a marker for good sleepers."

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.

"This study emphasises the importance of physical activity for children, not only for fitness, cardiovascular health and weight control, but also for sleep."

The research, published in the journal Archives of Disease in Childhood, also found that watching TV did not have any extra effect compared to a child who sat quietly reading.  Taking longer to fall asleep also was not linked to bad behavior nor was any significantly different sleep latencies seen for children who went to bed after 9pm.

The experts said other research has shown that 16% of parents of school-age children say their child has difficulty falling asleep. "This difficulty may promote poor sleep habits such as watching TV in bed or later bedtimes."

Mandy Gurney, founder of children's sleep clinic Millpond said, "Activity is not the be all and end all, and shouldn't be encouraged right before bedtime. What's essential is a routine wind-down hour, a quiet time before bed. A warm bath, but no longer than 10 minutes, and then straight into a darkened bedroom.

"That way you make the most of the natural sleep trigger of the warm water, and you can cap it all off with a bedtime story."