A new Swedish twin study found fresh insights, pointing to both genetics and shared environmental factors as key players in shaping infant sleep, crying, and the ability to settle, with influences that shifted between 2 and 5 months of age.
Researchers studied 921 twins at 2 and 5 months of age using parent-reported data. They assessed night wakings, time taken to settle during the day, evening, and night, and crying duration. The study also used polygenic scores to explore whether genetic risk for conditions such as autism or insomnia was linked to these early behaviors.
The study found a dynamic developmental picture. At 2 months, shared environmental factors had moderate-to-large influence on most sleep and settling behaviors, explaining 61 percent to 90 percent of the variation in nighttime awakenings. However, genetic influences increased substantially by 5 months, while shared environmental influences decreased for most behaviors except night wakings.
For nighttime awakenings specifically, shared environmental factors remained dominant at 5 months, but primarily represented the same environmental influences continuing from the 2-month timepoint (56 percent shared variance) rather than new environmental factors.
Settling ability showed the most dramatic developmental shift, becoming increasingly influenced by genetics over time. While shared environmental factors explained 36 percent to 65 percent of settling ability at 2 months, genetic factors dominated by 5 months, with heritability estimates between 51 percent and 67 percent.
Crying duration displayed moderate to high heritability at both ages across different times of day, with genetic factors explaining 29 percent to 70 percent of the variation. Evening crying at 5 months showed the highest heritability at 70 percent. Some genetic effects on crying and settling were stable across timepoints, but many were age-specific, reflecting rapid developmental changes during this critical period.
"A statistically significant positive association was found between polygenic score for autism and crying duration in the evening at 2 months, meaning that a higher polygenic score was related to longer periods of crying," wrote lead author Charlotte Viktorsson, MSc, of the Department of Psychology, Uppsala University, and colleagues.
No other polygenic scores—including those for insomnia, short or long sleep, ADHD, anxiety, or depression—were significantly associated with sleep or crying measures.
Investigators also observed "sibling interaction effects," where the behavior of one twin appeared to influence the other. For example, longer crying in one infant may have led to similar behavior in the co-twin. However, the authors note these effects might reflect either true behavioral influence between twins or parental rating bias, where parents unconsciously use one twin as a standard when rating the other's behavior.
The study's twin design allowed researchers to separate genetic from environmental influences, but relying on parent reports rather than objective sleep measures represents a limitation. Although the study did not assess parental sleep routines or maternal mental health—factors known to affect infant sleep—the authors identified these as important variables for future research.
The authors reported no conflicts of interest.
Source: JCPP Advances
