Medium-intensity light produces circadian rhythm adaptation to simulated night-shift work.

STUDY OBJECTIVES: To assess the effect of nocturnal light intensity on circadian adaptation to simulated night work.

SETTING AND PARTICIPANTS: Normal young men and women, simulated night work, home sleep.

DESIGN AND MEASUREMENTS: We compared temperature rhythm phase shifts following timed exposure to high (approximately 5700 lux 3 hours/day), medium (approximately 1230 lux 3 hours/day) or constant low-intensity (< 250 lux) light during consecutive night shifts. Subjects (n = 35) followed a schedule of 7 days baseline, 6 days of 8-hour night shifts (with day sleep delayed 10 hours from baseline sleep), and 4 days of recovery. Subjects wore dark sunglasses while outdoors during daylight. Sleep logs were completed after each 8-hour sleep/dark period. Night work fatigue was rated by questionnaire.

RESULTS: During the 3rd through 5th days of night work, most subjects in the high and medium groups (100% and 85%) exhibited phase delays large enough that their body temperature minima occurred within the daytime sleep/dark period. Only 42% of subjects in the low group exhibited phase delays large enough to meet this criterion of circadian adaptation. The phase shifts of the high and medium groups were not significantly different, and were significantly different from the low group. Larger phase shifts were correlated with more sleep and less fatigue.

CONCLUSIONS: Extremely "bright" light may not be necessary for circadian adaptation in shift work situations similar to our study protocol (e.g., regular daytime sleep/dark periods, sunglasses).