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

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 (> or = 8 shifts/ 15 days) were recruited from area hospitals. Following avacation period of > or = 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 approximately 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 (approximately 3,243 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.

Health-related interventions among night shift workers: a critical review of the literature

“…Follow-up ranged from 7-96 days (mean=23.7 days, median=14.0 days). The most common outcomes were sleep (N=9) (23-25, 29, 33-37) and markers of circadian rhythm: melatonin (N=7) (26-29, 32, 33, 35, 37, 38), cortisol (N=2) (31,32,38), and body temperature (N=3) (27,28,30,32,38). Controlled light exposure had different effects on health.…”

“…Cortisol was measured as an indicator of circadian rhythms in two studies; one was successful in shifting the usual release pattern of salivary cortisol (31), and the second increased plasma cortisol levels over the night shift (32,38). Body temperature also follows the circadian rhythm and was used to assess circadian adaptation to night shift work in three studies; two effectively altered body temperature (27,28,32,38), while one found no change (30). Other health outcomes evaluated are summarized in table 2.…”

Sleep and circadian misalignment for the hospitalist: A review

“…47,48 Light exposure ranged from 6 hours to 5 light treatments of 15 minutes each, with brightness ranging from 2,500 to 10,000 lux (approximating outdoor daylight; typical office lighting provides 200-500 lux). 47,49 Results demonstrate that bright light exposure during the night shift acutely improves alertness and performance, though not to daytime levels. 50 The greatest circadian adjustments occur in groups using both bright light during the night shift and light avoidance the following morning.…”

Light Treatment and Circadian Adaptation to Shift Work

“…These studies all indicate the heterogeneity of circadian adaptation to night shift work where progressive reduction of morning cortisol levels is consistent with some degree of circadian adaptation to a night oriented schedule 15,75) . Adaptive phase delays of cortisol and melatonin rhythms to an atypical work schedule are possible 76) and are influenced by the pattern of light and darkness exposure 17,75,[77][78][79][80][81][82][83][84] . The pattern of light exposure in night shift workers who spontaneously adapt to the shift work schedule are notable for the stability of sleep times 13) , limited exposure to bright sunlight in the morning 17) , brighter light during shift hours combined with sleep times in darkness 81) , and limited exposure to light throughout the day in Antarctic studies 85) .…”