BIOLOGICAL RHYTHMICITY IN NORMAL INFANTS DURING THE FIRST 3 MONTHS OFLIFE

Objective. The mammalian ''biological clock,'' which resides in the hy pothalamic 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 respir atory control. We wished to describe the development of the circadian system in normal infants during the first 3 months of life. Methods. T en healthy full term infants were studied in the infant's home for thr ee consecutive days at 1 month and 3 months postnatal age. Rectal temp erature, abdominal skin temperature, heart rate, and activity were rec orded at 2-minute intervals during each study using a small microcompu ter. Results. Circadian periodicity for most variables was seen at 1 m onth of age and significantly increased at 3 months. Differences in th e pattern of rhythmicity during these two developmental periods were h ighlighted by an increase in activity during the subjective day and a decrease in T-rec during the subjective night at 3 months compared to 1 month. Correlational analysis revealed that all pairs of variables, exclusive of T-sk, showed a significantly higher association at 3 mont hs relative to 1 month. The lengthening of the interfeeding interval a t 3 months of age corresponded with an increased consolidation of slee p during the night and a relatively lower nocturnal body temperature m inima compared to 1 month of age. Conclusion. The results of this stud y underscore the subtle changes in the nature and interaction of sever al infant variables during this critical developmental period, which m ay reflect maturation of the circadian system and its coupling with ho meostatic effector systems that are developing in parallel.