Control of sleep-wake states in the infant rat by features of the mother-infant relationship.

A series of experiments in 2 strains of Wistar rats explored which aspects of maternal separation were responsible for the sleep disturbance of the young. Neither their own mother, after mammary duct ligation, nor constant intragastric infusion of nutrient during the day of separation were found to ameliorate significantly the changes in sleep-wake state organization. However, a periodic nutrient delivery schedule and a formula closely resembling rat milk each ameliorated, and together entirely prevented, the sleep-wake disturbance. Decreased ambient temperature during separation tended to exaggerate the disturbance and periodic tactile-vestibular stimulation tended to reduce it. These findings suggest that the infant rat's normal sleep-wake pattern is maintained by the rhythmicity and composition of the milk delivered to it by its mother, although her warmth and behavioral interaction with it may play subsidiary roles. Separation produces its effects by removing these regulating features of the relationship.