How long a toddler sleeps at night depends in part on genes but environmental factors seem to make more of a difference for naps, a study of nearly 1,000 Canadian twins suggests.

Researchers asked parents of 405 identical and 586 fraternal infants born in the Montreal area to answer questions about daytime and nighttime sleep habits at ages six months, 18 months, 30 months and 48 months.

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Twin girls sleep. Canadian researchers studied the sleep habits of twins, with the results being published this week. (iStock)

"This study is the first to show that daytime sleep duration in early childhood is strongly influenced by environmental factors," Dr. Jacques Montplaisir from the University of Montreal and his colleagues concluded in Monday's issue of the journal Pediatrics.

At most ages, genetics accounted for between 47 and 58 per cent of nighttime sleep duration. The majority of children slept 10 or 11 continuous hours at night.

The exception was nighttime sleep at 18 months, which the researchers called "a critical environmental time-window" for establishing sleep patterns.

On the other hand, genes never explained more than about one-third of daytime nap time.

Environmental factors like family routines accounted for between 33 and 79 per cent of whether or not twins napped and for how long.

Fewer children were still napping by age four in this study, 4.3 per cent, compared with in an earlier Italian study, 68 per cent.  Montplaisir's team pointed to the difference as more evidence of the strong influence of environmental factors.

"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," Sonia Brescianini of the Italian Twin Register in Rome told Reuters. 

The Quebec researchers said the study needs to be repeated with more exact measurements of sleep, such as movement sensors or sleep lab experiments.

The study was funded by the Canadian Institutes of Health Research, Social Sciences and Humanities Research Council of Canada, Canada Research Chair Program, Canadian Institute of Advanced Research, National Health Research and Development Program, Quebec Research Funds, Quebec Ministries of Health, Social Services and Families, Lucie and André Chagnon Foundation, Ste-Justine Hospital, University of Montreal and Laval University.