The study, conducted by researchers at Rush University Medical Center, is the first to attempt to balance phase-delaying and phase-advancing light and dark exposure to achieve a “compromise circadian phase position” that delays the circadian clock to only partially align with the day sleep period. This could improve performance and alertness during night shifts and allow a sleep schedule that is compatible with both night shifts and days off.
“The major finding of this study was that complete physiological adaptation to a night shift and day sleep schedule does not appear necessary in order to improve night shift alertness and lengthen daytime sleep,” said lead author Mark Smith, post-doctoral fellow in the Biological Rhythms Research Laboratory at Rush University Medical Center. “Instead, we found that partial physiological adaptation using scheduled exposure to light and darkness is sufficient to bring night shift performance back to daytime levels.”
Twenty-four healthy subjects were included in the study with seven women and five men in both the experimental and control groups. All subjects underwent seven simulated night shifts from 11 p.m. to 7 a.m. with two days off. The goal was to delay the sleepiest circadian time for the experimental subjects so that it fell within the sleep episodes after night shifts and on days off, but to keep it from delaying too far beyond the scheduled sleep episodes on days off.
“Previous studies have shown the effectiveness of complete adjustment to a night shift schedule for improving nighttime alertness. But we think that most real shift workers want to be awake on their days off, and so would be unwilling to adopt a schedule that produces complete circadian adaptation because of the social constraints that are associated with it,” said Smith.
Those in the experimental group slept in dark bedrooms at scheduled times: 8:30 a.m. to 3:30 p.m. after the first two night shifts, from 8:30 a.m. to 1:30 p.m. after the third night shifts, from 3 a.m. to 12 p.m. on the two weekend days off, and again from 8:30 a.m. to 3:30 p.m. after the final four night shifts. They also were exposed to five, 15-minute, intermittent bright light pulses each night shift; wore dark sunglasses when outside; and received outdoor afternoon light exposure.
Control subjects, meanwhile, remained in normal room light during night shifts, wore lightly tinted sunglasses when outside, and had unrestricted sleep and outdoor light exposure.
Results show that performance was better for the experimental subjects than the controls. When the phase delays of the experimental group had likely reached the compromise circadian position, performance for this group was close to the level during day shifts, demonstrating fast reaction times with low variability and few or no lapses. In contrast, the control group continued to show longer and more variable reaction times on all night shifts.
The data suggest that in addition to adopting the recommended sleep pattern and wearing sunglasses, night-shift workers who use bright light exposure therapy will be able to alter their circadian rhythm in order to improve performance during night shift work, continue daytime interaction with peers and have the ability to sleep at night on days off.
For more information on how shift work may affect workers’ health, read Shift Work Linked to Organ Disease and WHO Study Links Night Shift Work, Firefighting to Cancer.