He may not have had enough exercise today...
Researchers have confirmed what parents have long believed - running around in the day means your child may well fall asleep faster at night.
But the study of 500 children provides a figure: for every hour they sit, they need three minutes longer to nod off.
Interestingly, it was not relevant what the child did while they sat. TV was no more detrimental than quietly reading.
And the Archives of Disease in Childhood found those who took longer to get to sleep were no worse behaved.
Experts from Monash University in Melbourne and the University of Auckland looked at 519 seven-year-olds.
The majority fell asleep within 45 minutes, and the average "sleep latency" - the time it took - was 26 minutes.
Children who were very physically active during the day tended to take less time to fall asleep, but the more prominent association was between being sedentary and taking longer to drift off.
Those who fell asleep faster also tended to sleep for longer. There has been much discussion about the impact of reduced sleep duration on children.
"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," the researchers wrote.
"This study emphasises the importance of physical activity for children, not only for fitness, cardiovascular health and weight control, but also for sleep."
They did not however find any evidence of bad behaviour, as measured by professional charts, among those who took longer to fall asleep. Nor did they find any significantly different sleep latencies for children who went to bed after 9pm.
Mandy Gurney, founder of children's sleep clinic Millpond, said the research was useful confirmation of the benefits of exercise for sleep as well as fitness and weight.
"But 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."