Appropriate use of bright light promotes a durable adaptation to night-shifts and accelerates readjustment during recovery after a period of night-shifts

Apr 1, 1995Work and stress

Using bright light helps people adjust better to night shifts and recover faster afterward

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Abstract

A maximum urinary aMT6s excretion shift of 7 hours was observed after exposure to bright light during night work.

Bright light exposure during night work may induce a significant phase delay in circadian rhythms.
This phase delay in urinary aMT6s excretion was stable across consecutive dim light night-work periods.
Improvement in the quality of daytime sleep and cognitive performance was noted for both groups following bright light exposure.
Complete synchronization of circadian rhythms occurred in five of six subjects during recovery after night shifts.
A single subject experienced partial synchronization, potentially due to limited exposure to light during the day.

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The present study examines the stability of bright light circadian readjustment during two consecutive dim light night-work periods and circadian synchronization during the recovery after a night-shift period. A sample of 10 subjects was divided into 2 groups (control group : 6 subjects; experimental group: 4 subjects). All subjects worked during 5 days, between 23:00 and 07:00 h and then went to sleep. Subjects received 2500-3000 lux between 02:00 and 05:00 h during 5 days for the control group and 3 days for the experimental group. During recovery after the night-shift periods, three cycles of bright light were administered at two different times: 12:00-15:00 h for two of six subjects from the control group and 10:00-13:00 h for all subjects (4) of the experimental group. By the fifth cycle of night-work the maximum of urinary aMT6s excretion that occurs at 05:00 h in the baseline condition was shifted to 12:00 h for the control and experimental groups (delay in hours: 7 +/- 1.6 (control); 7 +/- 1(experimental)). This result suggests that three cycles of bright light are sufficient to induce a significant phase delay and that this delay remained stable when night-work proceeded under dim light. The phase delay of the circadian aMT6s excretion by exposure to bright light was accompanied by an improvement of the quality of day sleep and level of cognitive and psychomotor performances for control and experimental groups. No significant difference was observed in the two groups for daytime sleep and nocturnal performance. The two bright light periods used during the three days of recovery induced a complete synchronization in five of six subjects. One subject showed a partial sychronization probably because he remained at the laboratory under dim light during the day and had few family and social contacts.

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Appropriate use of bright light promotes a durable adaptation to night-shifts and accelerates readjustment during recovery after a period of night-shifts

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