Babies who are good sleepers have both nature and nurture on their side, researchers say

If your child is a bad sleeper - one who can't fall asleep on her own, wakes up frequently at night, or insists on sleeping in your bed - is it because you are a bad parent? Or are her genes to blame?

For the most part, the fault probably lies with you and the choices you make about your child's sleeping environment, according to a study published online Monday in the Journal Pediatrics.

Italian researchers studied 127 pairs of identical twins and 187 pairs of fraternal twins to reach this conclusion. They interviewed parents about whether the twins - all 18 months old - slept with them or in their own room, whether the twins slept through the night most of the time, and whether they took daytime naps, among other things. All sets of twins were assumed to share the same environment (that's the "nurture" part of the nature-versus-nurture debate). The fraternal twins shared about half of their DNA, on average, while the identical twins shared virtually all of it (that's the "nature" part).

Here's what they found:

--Both identical and fraternal twins slept an average of 9.7 hours per night and napped for an additional 2.1 hours during the day.

--53 percent of identical and fraternal twins slept in the same room as their parents.

--"Environmental influences" (i.e. choices made by parents) were almost entirely responsible for whether the toddler twins were sleeping in their own room or with their parents. Genetics had nothing to do with it.

--Genes did play a role in whether the twins consistently woke up during the night. Though 31 percent of identical twins were reported to wake up at least seven times per week, only 19 percent of fraternal twins did so.

In addition, nearly 31 percent of the variation in the length of time twins slept at night was explained by genetics, along with 36 percent of the variation in length of nap time. Those figures are in the same ballpark as two previous pieces of twin-sleep research: A Japanese study that found genes accounted for 32 percent of the variation in time it takes to fall asleep, and a Canadian study that concluded genes were responsible for 44 percent of the variation in incidence of "sleep terrors."

The good news is that if your child is not a good sleeper, odds are you can improve the situation by changing his or her environment.

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