Adults can only envy the way a child can fall so completely and deeply asleep. It's like an on-off switch, going from being awake into profound sleep. I remember long summer holiday days in my own childhood, a city kid helping out on my grandfather's farm, tired and sunny, falling asleep before my head hit the pillow.
But there are plenty of kids who don't find it easy to fall asleep. My own children get thrown out of their regular pattern during the holidays and stay up too late and get too tired.
So what might help them to fall asleep? Part of the answer, according to research in the cheerfully-titled Archives of Disease in Childhood, is to run around a lot during the day.
Researchers have calculated the relationship between how much exercise children have during the day and the rapidity of falling asleep.
Studying a group of children with an average age of seven, researchers found that for every hour of sedentary activity - whether it was watching television or reading a book - an extra three minutes was added to the time it took children to fall asleep. Increasing the amount of physical exercise during the day meant a corresponding reduction in the amount of time it took to fall asleep.
So the answer is to run around all day for an instant sleep at night.