Paavonen EJ, Morales-Muñoz I, Pölkki P, et al. - In the present study, researchers characterized the normative variation in sleep-wake rhythm development in infancy, evaluated whether slower development is related to sleep quality and assessed factors associated with slower sleep-wake rhythm development. Questionnaires were available at the ages of 3 (n = 1,427) and 8 months (n = 1,302) and actigraph measurement at 8 months (n = 372). Infants with significant developmental delays (n = 11) have been excluded. Investigators discovered that the average percentage of daytime sleep was 36.3% at 3 months and 25.6% at 8 months. At both points of time, infants with slower sleep-wake rhythm development slept more hours per day, had a later sleep-wake rhythm, more problems in settling to sleep and longer sleep-onset latency. During the night, they also spent a longer time awake. The authors discovered that the infants with the slow development of a sleep-wake rhythm slept less and had a later start and end to night-time sleep than the other infants, according to actigraph registrations. The development of the sleep-wake rhythm of infants is highly variable and is related to the quality and quantity of sleep reported by parents and objectively measured. Interventions to improve the sleep-wake rhythm in these infants could improve the quality of sleep.