Genetics have a significant impact on how long kids sleep at night, however, environment may play a bigger role for naps in the afternoon, according to new research in the journal Pediatrics.

The study, conducted by researchers from the University of Montreal, found that half the variability in night-time sleep duration among children aged six months to 2 years was due to genetic influences.

The investigators aimed to examine the relative impact of environmental and genetic factors on night-time and daytime sleep duration at 6, 18, 30, and 48 months of age.

The study consisted of 995 twins (405 identical and 586 fraternal) from the Quebec Newborn Twin Study collected from the birth records of the Quebec Statistics Institute.

Night-time and Daytime sleep were analyzed through reports from the mother at six, 18, 30, and 48 months of age. The researchers were able to determine how much of the children’s sleep variation was due to genes (the same in identical twins) or environmental factors.

Among three of the four time points, genetics were responsible for between 47% and 58% of night-time sleep duration. The majority of kids slept continuously for 10 or 11 hours through the night.

Sleep was more fragmented than originally believed because video recordings revealed that many kids in one subgroup, even “good sleepers”, woke up approximately three times in one night.

As the twins aged, during naps, environmental influences explained more of the variation in sleep duration. These factors were responsible for:

  • 33% of nap duration at 18 months
  • 48% at 30 months
  • 79% at two years

Through age 2, nap duration remained stable and then decreased slowly over time with just 4% showing a faster than normal course. Just 4% stopped taking naps by the age of 2.

Slight to no variation in either night-time or daytime sleep was associated with environmental factors in just one twin, and not experienced by the other. However, the heritability estimate could have been influenced by using just one informant for the pair of twins.

Additionally, the use of ordinal categories for night-time and daytime sleep duration could be a limitation.

The authors conclude that this is the first established study showing that early childhood daytime sleep duration could be influenced by environmental settings, while the variance in consolidated night-time sleep duration is impacted mostly by genetic factors, with a significant environmental time-window influence at 18 months.

A separate study suggested a link between insufficient sleep time and increased risk for obesity in young children. Also, this research pointed out that napping is not an adequate substitute for night-time sleep in terms of preventing obesity.

Written by Kelly Fitzgerald