Development of circadian sleep-wake rhythms in preterm and full-term infants.

We have compared the roles of neurologic maturity and environmental time cues in the development of the entrained circadian sleep-wake rhythm in the preterm and term human infant. The preterm infants (n = 19) spent some time after birth in a hospital nursery with no environmental time cues, whereas the term infants (n = 22) were exposed from birth to a cyclical light and dark environment with one major caregiver. The circadian sleep-wake rhythm in the preterm infants entrained after a similar time of exposure to an environment with daily time cues but at an earlier postconceptional age when compared with the term group. We conclude, therefore, that it is the length of exposure to environmental time cues, rather than neurologic maturity, that determines the entrainment of the circadian rhythm of sleep and wakefulness in the human infant.