Genes play the biggest role in getting toddlers to sleep through the night, but environmental factors are more important for daytime naps, a twin study showed.
Genetic influences explained about half the variability in nighttime sleep duration from ages 6 months through 2 years, Jacques Montplaisir, MD, PhD, of Sacré-Coeur Hospital in Montreal, and colleagues reported in the June issue of Pediatrics.
There was a window of greater influence from surroundings and family habits at 18 months, which accounted for 48% of variability at that time. Environmental factors also accounted for up to almost 80% of variation in how long toddlers napped during the day by age 2.
Age 18 months may be a good opportunity to intervene for kids who don't sleep through the night, suggested Shalini Paruthi, MD, a pediatric sleep specialist at Saint Louis University in St. Louis.
"You really want to make sure your environment is as good as it can be," she told MedPage Today, which might include "keeping a dark and quiet room for them to sleep in and making sure they have their own sleep space so they're not bumping into a sibling on the mattress or on the bed."
"That doesn't mean every child in the same family is going to have the same sleep patterns," Paruthi noted, cautioning against overinterpretation.
The study also shouldn't be generalized to older children, she added.
Environment seems the biggest contributor to how long adults sleep at night, with a prior adult twin study suggesting less than 45% heritability.
Teens and pre-teens seem to follow adults in that pattern. However, young children haven't been followed longitudinally in the same way through the period when sleep is shifting toward a mature circadian rhythm.
Montplaisir's study included 995 sets of twins (405 identical) in the population-based Quebec Newborn Twin Study, which included pairs born without major medical conditions in the greater Montreal area from November 1995 through July 1998.
Mothers reported on their twins' sleep at 6, 18, 30, and 48 months of age, with a 2-week period between assessment for the two children to minimize homogenization of answers.
Genetics explained 47% of variability in how long kids slept without waking up their parents at 6 months of age, 58% at 30 months, and 54% at 48 months.
Sleep appeared to be more fragmented than parents were aware of, though, because video recordings in a subgroup showed that even "good sleepers" woke up three times a night on average.
With regard to daytime naps, environmental influences shared by the twins appeared to explain more of the variance in sleep duration as they got older.
These factors accounted for 33% of nap duration at 18 months of age, 48% at 30 months, and 79% at 2 years.
Nap duration seemed moderately stable through age 2 and declined gradually over time, with only 4% having a faster than normal trajectory in that regard.
Only 4% had stopped taking naps by age 2.
Little variation in either daytime or nighttime sleep appeared linked to environmental factors unique to one twin, not shared with the other.
However, the researchers cautioned that the heritability estimate may have been inflated by using a single "informant" for the twin pairs.
Another limitation was use of ordinal categories for daytime and nighttime sleep duration, so the study needs replication with quantitative measures, such as actigraphy or polysomnography, they noted.
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