The development of sleep-wakefulness rhythm in normal infants and young children.

The development of circadian sleep-wakefulness rhythm was investigated by a longitudinal study of two normal newborns for two and a half years and by a transversal study of 182 normal infants and young children living in three different areas of Japan. The circadian rhythm became established before 4 months of age, and daytime sleep became concentrated within two time periods from 7 months of age, and then within a single peak from 14 months of age. The time period 00:00-04:00 developed into the "absolute sleep period" from 3 months of age, and the "absolute wakefulness period" appeared first in the time period 08:00-11:00 from 14 months of age, and then in the time period 16:00-21:00 from one and a half years of age. Establishment of circadian rhythm as indicated by the numbers of sleep epochs in daytime and in nighttime, and the longest continuous sleep or wakefulness periods reflect the development of the nervous system and were influenced by the change of light-dark in the environment. These parameters showed critical ages, but not sexual or regional differences. However, regional differences were found in the times of waking up in morning and falling asleep at night. This indicated the phase of the circadian rhythm was also influenced by natural and social environmental factors.