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WASHINGTON – “Just one more story, please?” “I need a glass of water.” “I can’t sleep!”

When youngsters continually struggle to fall asleep at night, new research suggests that maybe their body clock doesn’t match their bedtime.

That doesn’t mean tots should be up at all hours.

“Just like nutrition and exercise, sleep is critical for good health,” said sleep scientist Monique LeBourgeois 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. Hint: It seems to have a lot to do with light.

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? A first-of-its-kind study tracked 14 healthy youngsters for six days to begin finding out.

The children, ages 2½ to 3, wore activity monitors 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 their chosen bedtime

For preschoolers, the study found that on average, their melatonin surge occurred at around 7:40 p.m. The children tended to be tucked in around 8:10 p.m., 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 hit the sack at about 8 fell asleep a bit faster. But when the melatonin surge was closer to bedtime, the youngsters were more likely to fuss.

Two children in the study actually were tucked in before their rise in melatonin ever occurred, and it took them up to an hour past bedtime to fall asleep, she said.

“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.

The study reinforces what doctors have long suspected is one bedtime barrier, said Dr. Jyoti Krishna, a pediatric sleep expert at the Cleveland Clinic. Other factors can disrupt a child’s sleep, but, he said, “this paper reminds us that, hey, there is a time that the body is more ready to sleep than at other times.”

The National Institutes of Health says preschoolers need 11 to 12 hours of sleep each day.

Krishna advises parents to look for cues – yawning, rubbing eyes – when setting a bedtime and then to adjust that bedtime as the child gets older.