Previous studies have emphasized the connection of sleep quality, length, and rhythmicity during infancy with brain maturation. There is, however, a lack of consensus regarding variables and methodology in relation to the diverse age periods. We thus conducted a longitudinal study with 136 healthy infants at ages 3, 6, and 12 months, expecting to capture maturation of the sleep-wake rhythm. We estimated circadian rhythmicity derived from actimetric data (Intradaily Variability, Interdaily Stability, Relative Amplitude and Circadian Function Index). Results confirm significant effects across ages. Intradaily Variability decreased with age, reflecting continuously less fragmented sleep and sleep-wake transitions. Interdaily Stability and Relative Amplitude increased with age, reflecting the gradual approaching to synchronization with the day-night cycle. More globally, the Circadian Function Index, averaging those three variables to indicate the maturation of the sleep-wake rhythm, illustrated that infants’ circadian status improves significantly from 3 to 12 months. These findings highlight the first year of life as a period of great changes in sleep-wake patterns and offer new normative data regarding estimates of circadian rhythm in infancy. Such maturation curves are fundamental to detect atypical development of sleep rhythm and early detection allows intervention for ameliorating negative development outcomes.