Exposure to Normal Indoor Light During Night Shifts Facilitates Circadian Adaptation

Introduction: Working at night is challenging because it can be difficult for the circadian system to adapt to a nocturnal schedule. The intensity of light exposure during the night-time is likely to be a major determinant of the rate of adaptation. The aim of this study was to examine the degree to which the circadian system adapts to a nocturnal schedule when exposed to different indoor ambient lighting conditions – normal (350 lux), moderate (100 lux), and dim (10 lux).

Methods: Participants (20 F, 30 M; age = 25.5 ± 4.9 year; mean ± SD) were randomly assigned to one of three conditions in a between-groups design. In each condition, participants spent 16 consecutive nights in the laboratory, including: (i) adaptation sleep, (ii) baseline sleep, and (iii) 14 consecutive nights shifts (19:00– 07:00h) followed by 7.5-hour sleep opportunities (09:30–17:00h). The only difference between conditions was the intensity of the indoor ambient light during the night shifts: dim light (10 lux; n = 17), moderate light (100 lux; n = 16), and normal light (350 lux; n = 17). Urine samples were collected during the baseline and daytime sleeps and subsequently assayed for urinary 6- sulphatoxymelatonin (aMT6s) by radioimmunoassay. Circadian adaptation was examined using the hourly rate of aMT6s production during sleep periods.

Results: There was a main effect of condition on the rate of aMT6s production during daytime sleep periods (F2,262=111; p < .001). Over the 14 consecutive night shifts, the concentration of aMT6s was higher in normal light condition compared with the moderate light condition (p < .001) and the dim light condition (p < .001). When expressed as a percentage of the total amount of aMT6s produced during sleep on the baseline night, the percentage of aMT6s after five consecutive nights shifts was 84% in the normal light condition, 40% in the moderate light condition, and 31% in the dim light condition; and the percentage of aMT6s after 14 consecutive nights shifts was 99% in the normal light condition, 57% in the moderate light condition, and 36% in the dim light condition.

Conclusion: The data indicate that people may adapt more quickly to night work when exposed to normal indoor light conditions during nights shifts (i.e., ~350 lux) than when exposed to less bright conditions during night shifts (i.e., ~10- 100 lux). The findings suggest that (i) longer sequences of night shifts may be suited to workplaces where shiftworkers operate in normal indoor light and are more likely to adapt to a nocturnal schedule; and (ii) shorter sequences of night shifts may be suited to workplaces where shiftworkers operate in dim light and are less likely to adapt to a nocturnal schedule. Support: This project was funded by the Australian Research Council.

Publication History

Article published online:
08 October 2025