For preschoolers, the new study found that on average, the melatonin surge occurred 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 around 8 fell asleep a bit faster. But when the melatonin surge was closer to bedtime, the youngsters were more likely to fuss or make curtain calls after lights out.
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, too, such as noise, stress or anxiety, or disrupted home routines, he cautioned.
“But this paper reminds us that, hey, there is a time that the body is more ready to sleep than at other times,” Krishna said.
The National Institutes of Health says preschoolers need 11 to 12 hours of sleep each day; some typically comes from an afternoon nap.
Parents don’t have melatonin tests as a guide, so Krishna advises looking for cues when setting a bedtime — yawning, rubbing eyes — and then to adjust that bedtime as the child gets older.
“The melatonin onset and our body rhythms change,” Krishna said. “You can’t stick to what worked two years ago with this child, because this child is now a different child.”
About 25 percent of young children experience some type of sleep difficulty, including trouble settling down at bedtime, LeBourgeois said. Harried parents aside, there’s concern that early-in-life bedtime frustration might lead to more persistent sleep trouble.