LAUSR

Circadian rhythms refer to biological rhythms that have an endogenous period length of approximately 24 hr. However, not much is known about the variance in the development of the sleep–wake rhythm. The study objectives were (a) to describe the normative variation in the development of a sleep–wake rhythm in infancy, (b) to assess whether slower development is related to sleep quality and (c) to evaluate factors that are related to the slower development of a sleep–wake rhythm. The study is based on a representative birth cohort. Questionnaires at the ages of 3 (n = 1,427) and 8 months (n = 1,302) and actigraph measurement at 8 months (n = 372) were available. Infants with significant developmental delays (n = 11) were excluded. The results are based on statistical testing and multivariate modelling. We found that the average percentage of daytime sleep was 36.3% (standard deviation [SD], 8.5%) at 3 months and 25.6% (SD, 6.6%) at 8 months. At both time‐points, infants with slower sleep–wake rhythm development slept more hours per day, had a later sleep–wake rhythm, more difficulties in settling to sleep and longer sleep‐onset latency; they also spent a longer time awake during the night. According to actigraph registrations, we found that the infants with slow development of a sleep–wake rhythm slept less and had a later start and end to night‐time sleep than the other infants. Infants’ sleep–wake rhythm development is highly variable and is related to parent‐reported and objectively measured sleep quality and quantity. Interventions to improve the sleep–wake rhythm might improve sleep quality in these infants.