Swedish researchers from Uppsala University conducted a large-scale study, examining data from almost a thousand twin infants to understand the nature of infant crying and the factors that influence babies' sleep.

The study involved 998 twins, both identical and fraternal. Parents filled out detailed questionnaires, reporting the frequency of nighttime awakenings, the duration of crying, and the time it took to fall back to sleep after each awakening.

The results were unexpected: the frequency of infant crying is largely determined by genetic factors. In two-month-old infants, heredity influences about half of the cases of crying, and by the age of five months, this proportion increases to 70%.

A completely different picture is observed with night awakenings - 90% of their number depends on external conditions. The daily routine and bedtime rituals play a decisive role in how often a child wakes up at night.

The speed of falling asleep showed a more complex relationship: at an early age it is 60% determined by environmental conditions, but by five months genetic factors begin to predominate, also accounting for about 60%.

The twin study was specifically chosen to test the hypothesis of a genetic predisposition to crying because this approach allows for a clear separation of the influence of heredity and environment.

Based on material from Victoria Priseko, MIR24.