Circadian Adaptation to Night-Shift Work by Judicious Light and Darkness Exposure

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Abstract
In this combined field and laboratory investigation, the authors tested the efficacy of an intervention designed to promote circadian adaptation to night-shift work. Fifteen nurses working permanent night schedules ([.greaterequal] 8 shifts/ 15 days) were recruited from area hospitals. Following a vacation period of [.greaterequal] 10 days on a regular daytime schedule, workers were admitted to the laboratory for the assessment of circadian phase via a 36-h constant routine. They returned to work ~ 12 night shifts on their regular schedules under one of two conditions. Treatment group workers (n = 10, mean age ± SD = 41.7 ± 8.8 years) received an intervention including 6 h of intermittent bright-light exposure in the workplace (~ 3243 lux) and shielding from bright morning outdoor light with tinted goggles (15% visual light transmission). Control group workers (n = 9, mean age ± SD = 42.0 ± 7.2 years) were observed in their habitual work environments. On work days, participants maintained regular sleep/wake schedules including a single 8-h sleep/darkness episode beginning 2 h after the end of the night shift. A second 36-h constant routine was performed following the series of night shifts. In the presence of the intervention, circadian rhythms of core body temperature and salivary melatonin cycles were delayed by an average (± SEM) of –9.32 ± 1.06 h and –11.31 ± 1.13 h, respectively. These were significantly greater than the phase delays of –4.09 ± 1.94 h and –5.08 ± 2.32 h displayed by the control group (p = 0.03 and p = 0.02, respectively). The phase angle between circadian markers and the shifted schedule was reestablished to its baseline position only in the treatment group of workers. These results support the efficacy of a practical intervention for promoting circadian adaptation to night-shift work under field conditions. They also underline the importance of controlling the overall pattern of exposure to light and darkness in circadian adaptation to shifted sleep/wake schedules.