The spontaneous sleep‐wakefulness behavior of three infants (one male and two females) from the same parents, normal with respect to pregnancy and delivery, was observed continuously from 2–14 days after birth up to 4 months of life. The infants were allowed to feed and sleep according to their own schedule. One of them supplied another sample of clear freerunning circadian rhythmicity during the first postnatal month. The least square cosine fitting method and the maximal entropy spectral analysis with high spectral resolution revealed the circadian rhythmicity to exist as early as the first week of life. This fact may suggest that the human circadian clock regulating the daily sleep‐wakefulness rhythm is already functional at birth. It was also found that sleep duration depended on the timing of sleep: the first nighttime sleep being the longest, while the sleep during the early morning and the late afternoon was the shortest. These results are discussed with respect to the development of circadian rhythmicity.