Research out of the University of Montreal and that was recently published in Pediatrics explored the link between genetics and sleep duration in children from ages six months to two years. The research team examined both daytime and nighttime sleep duration using statistics from pairs of twins and found that both nature and nurture account for nearly equal influence when it comes to children’s sleep duration.

The research is based on data obtained from the Quebec Newborn Twin Study via records at the Quebec Statistics Institute. The team analyzed 405 identical twins and 568 fraternal twins, evaluating sleep duration reported for children aged 6, 18, 30, and 48 months.

They worked to discover how much of the variations in the children’s sleep was from genetic factors and how much was from environmental factors. The results showed that genetics accounted for between 47 to 58 percent of night-time sleep duration and that the majority of children slept continuously for 10 or 11 hours at night.

However, the researchers also scrutinized daytime sleep duration patterns—napping specifically—where they found that environmental factors played a much bigger role. Environmental influences became increasingly important to the length of daytime naps as children aged, accounting for 33% of nap duration at 18 months, 48% at 30 months, and 79% at two years. Nap duration slowly decreases as children age, but only 4% of children had stopped taking naps altogether by age two.

This is the first study revealing information about the question of nature versus nurture in sleep duration. Genetic factors had the biggest impact on nighttime sleep duration, while daytime sleep duration is more likely to be influenced by the environment. The effects of environmental factors increase at 18 months.