Twin study reveals genetic influence on infant crying and sleep

How much a baby cries is largely determined by its genetics, and there's probably little parents can do about it. That's according to a new Swedish twin study by scientists at Uppsala University and Karolinska Institutet, in which researchers looked at how genes and environment influence how long babies cry, how well they sleep, and how well they can self-soothe in the first months of life.

The study, recently published in JCPP Advances, is based on questionnaires filled out by parents of 1,000 twins across Sweden. The parents were asked about their babies’ sleep, crying, and self-soothing when the twins were 2 months old and again when they were 5 months old. The researchers were interested in how genes and environment influence these behaviors in the first months of life—something no study has done before.

The clearest results were obtained when the researchers analyzed how much time per day the children cried.

“We found that crying is largely genetic. At 2 months, genes explain about 50% of how much a baby cries. At 5 months, genetics explain up to 70% of the variance. It can be comforting for parents to know that their baby’s crying is largely explained by genetics and that they have limited control over how much their baby cries,”
says Charlotte Viktorsson, a postdoctoral fellow in psychology and lead author of the study.

The remaining percentage that cannot be explained by genes, scientists attributed to what they call the “unique environment” - these are factors in the child’s environment or life situation that are unique to each person and cannot be accurately identified by questionnaires.

Twin studies show importance of genetics

Participants in the study were recruited through letters sent to families with twins aged 1–2 months. These families were selected from a population registry. To determine the extent to which behavior was determined by genes, the researchers compared identical (monozygotic) twins with fraternal (dizygotic) twins. The advantage of studying twins is that they share important factors such as home environment, family situation, and socioeconomic status. If identical twins are more similar to each other than fraternal twins on a trait (such as how much they cry), this is taken as evidence that genetics are important for that trait.

Environment Plays a Role in Babies' Sleep Time

Using the same method, the researchers analyzed the number of times the children woke up at night. Here, genes played a smaller role. The number of nighttime awakenings was largely determined by environmental factors, such as sleep patterns and the conditions in which the child sleeps. In questionnaires, parents also indicated how much time passed from the moment the child was put down until he or she fell asleep.

"Sleep onset speed at 2 months was primarily influenced by the environment, but by 5 months genes began to play a role. This reflects the rapid development of infants and may indicate that parents' efforts to get their baby to sleep have the greatest effect in the first months,"
notes Charlotte Viktorsson.

However, it is difficult to draw conclusions about which interventions are effective based on an observational study.

"While we cannot determine which specific environmental factors influence the number of nighttime awakenings or the time it takes to fall asleep, our study provides direction for future work that will focus on sleep patterns," she adds.

The researchers continued to follow the twins until they were 36 months old, allowing them to see how their sleep and crying changed as they got older. This study is the first in a series to build on this data.

Facts about the study

Parents in the study were asked questions about how much their babies cried, how often they woke up during the night, and how long it took them to fall asleep. There was a lot of individual variation between babies. For example, some babies might wake up as many as 10 times a night. The averages are below:

2 months:

• Duration of crying (in 24 hours): about 72 minutes
• Awakenings: 2.2 times per night
• Time to sleep: about 20 minutes

5 months:

• Duration of crying (in 24 hours): about 47 minutes
• Awakenings: 2.1 times per night
• Time to fall asleep: about 14 minutes