The development of ultradian and circadian rhythms in premature babies maintained in constant conditions.

Twenty very premature babies, born at 24-29 weeks gestation, have been studied while they were maintained in intensive care with continuous intravenous feeding and constant ambient lighting and temperature. Hourly records of insulated skin temperature and heart rate were made for a continuous period of 6-17 weeks, always starting the recording within 24 h of birth. The development of rhythms within the ultradian, circadian and infradian domains was sought by methods including maximum entropy spectral analysis and autocorrelation. Circadian and ultradian rhythms were present, but not regularly so; rather they appeared and disappeared erratically in successive weeks. As a consequence, the group as a whole did not show an increasing rhythmicity with chronological age. In some cases, babies were later placed in a ward in which the lighting was dimmed at night, and feeding by mouth at regular intervals was instituted. There was some evidence for increases in circadian and ultradian rhythmicity after these changes. These results enable inferences to be drawn as to the origin of fetal rhythms in the third trimester of pregnancy, as well as speculation to be made on the ontogeny of ultradian and circadian rhythms in the neonate.