Regulation of Sleep Rate and Orcadian Consolidation of Sleep and Wakefulness in an Infant

Observations of sleep and wakefulness were continuously made over an infant's first 6 months by Kleitman and Engelmann. Those data have been reanalyzed using a newly developed tool, fractional-time analysis, and two measures derived from it: 1) sleep rate, the rate at which the infant accumulated sleep; and 2) consolidation, the degree to which episodes of sleep and wakefulness were sustained and clustered. Changes in sleep rate and consolidation were independent and were attributed to separate underlying neural mechanisms. Consolidation increased over the 6-month course of development. This was explained by the development of circadian rhythmicity, which appeared during the 1st month as a free-running, endogenously generated rhythm in sleep-episode durations. Starting in the 2nd month, sleep rate was regulated, and it remained remarkably stable for weeks at a time. Sleep rate decreased over the 6-month period, perhaps because metabolic rate decreased as the infant grew. Short-term (weeks-long) changes in consolidation and sleep rate were explained by "beating" of the non-24-hour, endogenous sleep rhythm with direct effects of the 24-hour environment on sleep and wakefulness. Environmental stimuli eventually entrained the circadian pacemaker, bringing the consolidating periods of wakefulness and sleep into register with day and night. The infant was thereby efficiently exposed to periodic sensory and social stimuli, promoting normal neural development. A model of sleep-wake regulation based on independent mechanisms for circadian timing and sleep-wake regulation is proposed.