The bedtime parents select for their toddler may be out of sync with the child’s internal body clock, potentially contributing to sleep disorders later in life, according to a new University of Colorado (CU) Boulder study.

The study, published this month in the journal Mind, Brain and Education, 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, says 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,” says LeBourgeois, a faculty member in the integrative physiology department who led the new study, in a release. “Sleeping at the wrong ‘biological clock’ time leads to sleep difficulties, like insomnia, in adults.”

While adults get to choose their own bedtimes, toddlers rarely have this option. “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,” LeBourgeois says.

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 says.

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.

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 says.

For the study, the research team recruited 14 families in Providence, RI, each of which had a child between 30 and 36 months old who slept at least 10.5 hours nightly and took a daytime nap of at least 45 minutes. Saliva samples containing the children’s melatonin levels were collected every 30 minutes over a six-hour period on one evening before bedtime.

The average evening melatonin onset for the toddlers occurred at roughly 7:40 pm, which occurred about 30 minutes before parent-selected bedtimes, LeBourgeois says. On average, the toddlers fell asleep about 30 minutes after bedtime.

The study showed several toddlers who were put to bed before their rise in melatonin took 40-60 minutes to fall asleep. “For these toddlers, lying in bed awake for such a long time can lead to the association of bed with arousal, not sleep,” she says. “This type of response may increase children’s lifelong risk for insomnia over time.”