Objectives. Many hospitalized premature infants are exposed to continuous dim lighting rather than to cycled lighting. However, we do not know whether dim lighting or low-intensity cycled lighting is more conducive to the development of rest-activity patterns that are in phase with the solar light-dark cycle. Thus, we examined the effects of nursery lighting conditions on the development of activity patterns in premature infants.

Methods. Premature infants who were born at <32 weeks’ postmenstrual age and were medically stable in neonatal intensive care unit rooms were randomly assigned between 32 and 34 weeks’ postmenstrual age to either continuous dim lighting (<25 lux; duration 24 days; control group; n = 29) or cycled lighting (239 ± 29 lux, 7:00 am to 7:00 pm; <25 lux, 7:00 pm to 7:00 am; duration: 25 days; experimental group; n = 33). Activity was continuously monitored from enrollment until approximately 1 month after discharge from the hospital. Weight and head circumference were also assessed up to 6 months after discharge from the hospital.

Results. Over the first 10 days at home, distinct day-night differences in activity were not seen in control subjects (D day-night: N 1.07 ± 0.02), but experimental group infants were more active during the day than at night (day-night: 1.25 ± 0.03). It was not until 21 to 30 days after discharge that day-night activity ratios in control infants matched those seen in experimental group infants shortly after discharge, yet even at this age, experimental group infants (day-night: 2.13 ± 0.19) were considerably more active during the day than at night as compared with control subjects (day-night: 1.43 ± 0.09).

Conclusion. Exposure of premature infants to low-intensity cycled lighting in the hospital nursery induces distinct patterns of rest-activity that are apparent within 1 week after discharge. In comparison, the appearance of distinct patterns of rest and activity are delayed in infants who are exposed to continuous dim lighting in the hospital. These observations show that day-night rhythms in activity patterns can be detected shortly after discharge to home in premature infants and that the circadian clock of developing infants is entrained by cycled lighting.

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