Circadian Rhythm Adaptation to Simulated Night Shift Work: Effect of Nocturnal Bright-Light Duration

Jul 1, 1995Sleep

How the Body Clock Adjusts to Night Shift Work Depends on How Long Nighttime Bright Light Is Used

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Abstract

Substantial circadian adaptation was observed in many subjects exposed to bright light for 6 or 3 hours during night shifts.

Bright light exposure (about 5,000 lux) during night shifts resulted in a significant phase shift of the circadian temperature rhythm compared to dim light (< 500 lux).
Both 6 and 3 hours of bright light were effective in producing temperature rhythm phase shifts, with no significant difference between the two durations.
A duration of bright light exposure longer than 3 hours may not be necessary for effective phase shifting in similar work conditions.
Larger phase shifts in temperature rhythm were linked to improved daytime sleep quality, reduced fatigue, and enhanced overall mood.

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We compared bright-light durations of 6, 3 and 0 hours (i.e. dim light) during simulated night shifts for phase shifting the circadian rectal temperature rhythm to align with a 12-hour shift of the sleep schedule. After 10 baseline days there were 8 consecutive night-work, day-sleep days, with 8-hour sleep (dark) periods. The bright light (about 5,000 lux, around the baseline temperature minimum) was used during all 8 night shifts, and dim light was < 500 lux. This was a field study in which subjects (n = 46) went outside after the night shifts and slept at home. Substantial circadian adaptation (i.e. a large cumulative temperature rhythm phase shift) was produced in many subjects in the bright light groups, but not in the dim light group. Six and 3 hours of bright light were each significantly better than dim light for phase shifting the temperature rhythm, but there was no significant difference between 6 and 3 hours. Thus, durations > 3 hours are probably not necessary in similar shift-work situations. Larger temperature rhythm phase shifts were associated with better subjective daytime sleep, less subjective fatigue and better overall mood.

Circadian Rhythm Adaptation to Simulated Night Shift Work: Effect of Nocturnal Bright-Light Duration

Abstract

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