Dec. 16, 2013 The bedtime you select for your toddler may be out of sync with his or her internal body clock, which can contribute to difficulties for youngsters attempting to settle in for the night, according to a new University of Colorado Boulder study.

The study pinpointed the time when the hormone melatonin increased in the evening, indicating the start of the biological night, in a group of 14 toddlers whose sleep also was studied over the course of six days. The study showed that toddlers with later melatonin rise times took longer to fall asleep after being put to bed, said CU-Boulder Assistant Professor Monique LeBourgeois.

"There is relatively little research out there on how the physiology of toddlers may contribute to the emergence of sleep problems," said LeBourgeois, a faculty member in the integrative physiology department who led the new study. "Sleeping at the wrong 'biological clock' time leads to sleep difficulties, like insomnia, in adults."

While adults get to choose their own bedtime, toddlers rarely have this option, said LeBourgeois. "This study is the first to show that a poor fit between bedtimes selected by the parents of toddlers and the rise in their evening melatonin production increases their likelihood of nighttime settling difficulties," said LeBourgeois.

The findings are important because about 25 percent of toddlers and preschoolers have problems settling after bedtime, said LeBourgeois. Evening sleep disturbance can include difficulties falling asleep, bedtime resistance, tantrums, and episodes known as "curtain calls" that manifest themselves as calling out from bed or coming out of the bedroom, often repeatedly, for another story, glass of water or bathroom trip, she said.

Toddlers with longer intervals between the onset of nightly melatonin release and their subsequent bedtimes were shown to fall asleep more quickly and had decreased bedtime resistance as reported by their parents, according to the study.

A paper on the subject was published this month in the journal Mind, Brain and Education. Co-authors included University Children's Hospital Zurich Director of Child Development Oskar Jenni and CU-Boulder Associate Professor Kenneth Wright Jr. The National Institute of Mental Health funded the study.

Sleep problems in early childhood are predictive of later emotional and behavioral problems, as well as poor cognitive function, that can persist into later childhood and adolescence. In addition, parents of young children with sleep problems often report increased difficulties in their own sleep patterns, which can cause chronic fatigue and even marital discord, she said.

"A natural next step is to optimize our knowledge of the interactions between physiology and the environment to further understand how problems like bedtime resistance first develop and how they are maintained," LeBourgeois said.

Research in adolescents and adults has shown that exposure to light in the evening can delay the timing onset of melatonin. Whether the later rise of melatonin in some toddlers can be pushed to an earlier time by restricting evening light or by increasing morning light exposure is a question still to be answered, she said.

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Bedtime for toddlers: Timing is everything