The emergence of the sleep-wake cycle in infancy.

In order to investigate the appearance of a rest-activity periodicity, spontaneous motor behavior of five low-risk preterm infants (gestational age range 27-31 weeks) was monitored for 24 hours at weekly intervals until term. Using a time lapse videorecorder (6.5 fields/sec.) both slow and rapid movements were counted every three minutes; this interval was adopted as basic chronological paradigm in order to evaluate the daily distribution of motor activity. The results of quantitative analysis showed a developmental trend of motor behavior which increased from earliest conceptional ages (C.A.) until 32-33 weeks, and decreased around 36-37 weeks C.A. Prolonged complete rest intervals are rare at low C.A., while in older infants (approx. 40 weeks C.A.) an initial tendency to organize motor activity in ultradian cycle-like periods seems to be more evident, contemporary with consolidated behavioral states.