12/18/2013 04:06 pm ET Updated Jan 23, 2014

Why The Right Bedtime For A Toddler Is So Important

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If your toddler fights falling asleep, it may not necessarily be willfulness, lack of discipline or any of the other culprits frequently cited by health care providers and parenting experts. A new study suggests that the bedtimes parents set for their toddlers may simply be out of sync with their internal body clocks.

Researchers with the University of Colorado Boulder recruited 14 families with children between ages 2 1/2 and 3. They collected saliva samples every 30 minutes for the six hours prior to bedtime, in order to track the toddlers' melatonin levels. Melatonin is a hormone that helps regulate individuals' natural sleep-wake schedules. It rises in the evening and stays high throughout the night to help promote sleep.

The researchers also tracked what time the toddlers went to bed, relying on the reporting of parents as well as a device the toddlers wore on their wrists, which tracked when they were put to bed and when they actually fell asleep.

The average time of melatonin onset -- meaning, "the time when the hands on the internal biological clock say, let's get ready for sleep ... not necessarily go to sleep yet, but be prepared for sleep," explained study researcher Monique LeBourgeois -- was 7:40 p.m. On average, that was roughly 30 minutes before parent-selected bedtimes.

"Thirty minutes is a pretty good interval," added LeBourgeois, who is an assistant professor in the department of Integrative Physiology at Boulder.

"If it's anything under 30 minutes, those kids are much more likely to have problems falling asleep," she continued. "The longer the interval between melatonin onset and the bedtime the parents selected, the shorter the time it took them to fall asleep."

That is not to say, however, that some kids are somehow "biologically adverse to early bedtimes," she said. "That's a terrible message." Parents should simply be aware that their child might have a later internal clock, which they can gauge by tracking when they tend to fall asleep and awake, left to their natural devices. If they sense their toddlers' natural body clock is out of sync with their bedtime, parents can take steps to help shift it earlier. One strategy is limiting exposure to light before bed, which directly affects melatonin levels.

"In today's society, with iPads and computers and TV, even in toddlers it's not uncommon to have this exposure to electronic devices and light at night," LeBourgeois explained. "Light, in the evening, delays the clock."

Another key, said Dr. Shalini Paruthi, a spokeswoman for the American Academy of Sleep Medicine, is maintaining a set wake time -- even on weekends. Most toddlers really do require between nine and 10 hours of sleep per night, said Paruthi, and will typically fall into a set sleep pattern within two weeks. "If they wake up very consistently at 6:30 a.m., and aren't able to fall asleep around 8:30 or 9, you probably do need to look closer at what's happening," she said, explaining that other factors, like sleep apnea or restless leg syndrome, might be at play.

As for "curtain calls" -- those late night pleas for water, or hugs, which can exacerbate sleep problems, because toddlers are exposed to more light when they head into their parents' rooms -- Paruthi advocates a simple reward system. Children are given two bedtime passes when they're tucked in at night. Each time they wake and seek out their parents, they have to hand over a pass. But if they wake in the morning with one or both of their passes in hand, parents give them a small reward, such as stickers or gummy snacks.

And when it comes to toddlers and sleep, parents need all the help they can get, LeBourgeois said, joking that the wildly popular "Go The F*** To Sleep" was the poster book for her study, which was published in the December issue of the journal Mind, Brain, and Education.

"Those 'curtain calls' are a major problem for parents and kids, and it increases across early childhood. By age 5, up to 50 percent of kids are showing bedtime resistance," LeBourgeois said. "What's driving that? Is it the bedtime routine? That there's nothing reinforcing about sleep for little kids? I think all of those things are important, but this study's the first to say that physiology also plays a really important role."