Tots' Sleep Differences Due to Genes, Environment, Study Suggests

But parents should still try to correct bad sleep habits, expert says

By Amy Norton

HealthDay Reporter

MONDAY, May 27 (HealthDay News) -- A new study of twins suggests that genes may play a big role in how long babies and toddlers sleep at night, while environment is key during nap time.

Researchers found that among nearly 1,000 twins they followed to age 4, genes seemed to explain much of the difference among youngsters' nighttime sleep habits. In contrast, napping seemed mainly dependent on the environmental setting -- especially for toddlers and preschoolers.

So does this mean the amount of sleep your little one gets at night is out of your control?

No, said the lead researcher on the study, which was published online May 27 in the journal Pediatrics.

"[Parents] should not give up on trying to correct inadequate sleep duration or bad sleep habits early in childhood," said Evelyne Touchette, of Laval University in Quebec, Canada.

For one, the study found that environment did matter in babies' and toddlers' nighttime sleep -- and even seemed to overshadow genes by the age of 18 months.

The reasons for the findings are unclear, Touchette said. But she said it makes sense that environment would matter more at the age of 18 months versus 6 months, when the maturation of the brain may be key in infants' ability to sleep for longer stretches at night.

There's no clear explanation, though, for why genetic influences became stronger again after the age of 18 months, Touchette said.

A sleep researcher not involved in the study said it's not really possible to break down children's sleep into "nature or nurture" questions.

"Everything is a complex interaction between genes and environment," said Hawley Montgomery-Downs, an associate professor of psychology at West Virginia University in Morgantown.

It's not possible, she said, to parse out what proportion of young children's sleep duration is due to genes, and what proportion is environment.

For the study, Touchette's team followed nearly 1,000 Canadian twins whose mothers reported on their sleep habits from the ages of 6 months through 4 years. About 400 children were identical twins, which means both twins share all of the same genes; the rest were fraternal twins, who are no more genetically similar than non-twin siblings.

In general, such studies can help researchers sort out the influences of genes versus "shared environment," which could include anything from a mom's diet during pregnancy to family income.

When it came to hours slept at night, genes seemed to explain more than half of the variance among children at the ages of 30 months and 4 years. Genes were nearly as important at the age of 6 months.

The exception was the age of 18 months, when environment seemed to account for about half of the variance among the children.

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