That doesn't mean children should be up at all hours. "Just like nutrition and exercise, sleep is critical for good health," said sleep scientist Monique Le Bourgeois of the University of Colorado, Boulder, who is leading the research.
The ultimate goal is to help reset a delayed sleep clock so that young children can settle down more easily, she said.
We all have what's called a circadian rhythm, a master biological clock, that regulates when we become sleepy and when we're more alert. Those patterns vary with age. It's the reason teenagers are notorious for late nights and difficult-to-wake mornings.
But how does that clock work in preschoolers, who need more sleep than older kids or adults? A first-of-its-kind study tracked 14 healthy youngsters for six days to begin finding out.
The children, aged 2-and-half to 3, wore activity monitors on their wrists to detect when they slept. Parents kept diaries about bedtime routines.
Then on the last afternoon, researchers visited each home, dimming lights and covering windows. Then every 30 minutes for six hours leading up to the child's appointed bedtime, they also coaxed each tot to chew on some dental cotton to provide a sample of saliva.
The reason: To test for levels of a hormone named melatonin that is key to the sleep cycle and also sensitive to light. At some point every evening, people's melatonin levels surge and a while later, they begin to feel sleepy.

Among adults who sleep well, that melatonin rise tends to happen about two hours before whatever is their chosen bedtime.
For preschoolers, the new study found that on average, the melatonin surge occurred around 7:40 pm. The children tended to be put to bed around 8:10 pm, and most were asleep 30 minutes later, LeBourgeois reported in the journal Mind, Brain and Education.
When melatonin rose earlier in the evening, tots who went to bed around 8 fell asleep a bit faster. But when the melatonin surge was closer to bedtime, the youngsters were more likely to fuss after lights-out.

"We don't know what that sweet spot is yet," LeBourgeois said, but the data suggest bedtime is easiest if the melatonin surge occurred at least 30 minutes earlier.


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