Biological Rhythmicity in Normal infants During the First 3 Months of Life

Abstract

The mammalian "biological clock," which resides in the hypothalamic suprachiasmatic nucleus, has an important role in both the timing and organization of sleep and in the coordination of sleep with other physiological rhythms such as temperature regulation and respiratory control. We wished to describe the development of the circadian system in normal infants during the first 3 months of life. Ten healthy full term infants were studied in the infant's home for three consecutive days at 1 month and 3 months postnatal age. Rectal temperature, abdominal skin temperature, heart rate, and activity were recorded at 2-minute intervals during each study using a small microcomputer. Circadian periodicity for most variables was seen at 1 month of age and significantly increased at 3 months. Differences in the pattern of rhythmicity during these two developmental periods were highlighted by an increase in activity during the subjective day and a decrease in Trec during the subjective night at 3 months compared to 1 month. Correlational analysis revealed that all pairs of variables, exclusive of Tsk, showed a significantly higher association at 3 months relative to 1 month. The lengthening of the interfeeding interval at 3 months of age corresponded with an increased consolidation of sleep during the night and a relatively lower nocturnal body temperature minima compared to 1 month of age. The results of this study underscore the subtle changes in the nature and interaction of several infant variables during this critical developmental period, which may reflect maturation of the circadian system and its coupling with homeostatic effector systems that are developing in parallel.