A new study based on pairs of Canadian twins suggests that genetics play a strong role in how long kids sleep at night, but their environment may be more important for afternoon naps.
Researchers compared sleep patterns for close to 500 sets of identical or fraternal twins. They found that especially as kids got older, environmental influences - which include when parents put kids in bed, for example - explained more and more of their napping differences.
"Even though the results do not explicitly state which are the environmental factors," said Sonia Brescianini, "researchers in the field think that familial habits at bed time, sleeping routines, and in general sleep hygiene measures are for sure a means of improving sleep in children."
Brescianini, a researcher with the Italian Twin Register at Rome's Istituto Superiore di Sanità who was not involved in the new study, said in an email that past research has also suggested a child's environment may be more important than genetics for daytime sleep habits.
The new study involved 983 individual children born in greater Montreal, Canada, from 1995 to 1998. Mothers were asked about each twin's daytime and nighttime sleep patterns at age 6, 18, 30 and 48 months.
By comparing sets of identical and fraternal twins, Dr. Jacques Montplaisir from the University of Montreal and his colleagues were able to determine how much of kids' sleep variation was due to genes - which are the same in identical twins - and how much to environmental factors.
For example, Brescianini explained, the more identical twins are similar and fraternal twins are not for a given trait such as sleep schedules, the more that trait is likely to be genetic. But if both types of twins resemble each other in similar ways, the trait is likely affected by a child's environment.
At three of the four time points, genetics accounted for between 47 and 58 percent of nighttime sleep duration. The majority of children slept 10 or 11 continuous hours at night.
The one age when environment seemed to play a strong role in overnight sleep was at 18 months. That suggests that age "might be an appropriate time of intervention in children with not satisfying sleep duration and quality," Brescianini said.
But genes never explained more than about one-third of daytime nap length, the researchers reported Monday in Pediatrics.
Family routines and other shared environmental influences, on the other hand, accounted for between 33 and 79 percent of whether or not twins were napping during the day, and for how long. That association became stronger as children got older and average nap length shortened.
The researchers said a similar study needs to be done using more exact tools to measure sleep, such as movement sensors or laboratory monitors.
In addition, it will be helpful to look at what family-related factors may affect daytime sleep, Montplaisir and his colleagues wrote.
"Good sleep duration and quality is very important for children's physical and mental health because sleeping behavior has been associated with obesity, poor school performance and other behavioral disorders," Brescianini said.
"In order to promptly intervene, more knowledge is needed on how sleeping behavior changes through childhood."