Shift work: health, performance and safety problems, traditional countermeasures, and innovative management strategies to reduce circadian misalignment

Abstract

There are three mechanisms that may contribute to the health, performance, and safety problems associated with night-shift work: (1) circadian misalignment between the internal circadian clock and activities such as work, sleep, and eating, (2) chronic, partial sleep deprivation, and (3) melatonin suppression by light at night. The typical countermeasures, such as caffeine, naps, and melatonin (for its sleep-promoting effect), along with education about sleep and circadian rhythms, are the components of most fatigue risk-management plans. We contend that these, while better than nothing, are not enough because they do not address the underlying cause of the problems, which is circadian misalignment. We explain how to reset (phase-shift) the circadian clock to partially align with the night-work, day-sleep schedule, and thus reduce circadian misalignment while preserving sleep and functioning on days off. This involves controlling light and dark using outdoor light exposure, sunglasses, sleep in the dark, and a little bright light during night work. We present a diagram of a sleep-and-light schedule to reduce circadian misalignment in permanent night work, or a rotation between evenings and nights, and give practical advice on how to implement this type of plan.

Keywords: bright light; circadian rhythms; melatonin; night work; phase-shifting; sleep.

Figure 1

Sleep-and-light schedule for night-shift work that we tested to determine if it could align circadian rhythms with the sleep schedule enough to move the temperature minimum (Tmin) to within sleep. Notes: Blue rectangles represent scheduled sleep times for experimental subjects. They slept at home, in bedrooms that we made completely dark, and “worked” their night shifts in the lab. They were exposed to intermittent bright light (~4000 lux) during the night shifts (yellow rectangles), timed to delay their circadian rhythms. They wore sunglasses (~15% visual light transmission) to reduce the intensity of phase-advancing daylight when traveling home from the night shift. They were required to go outside within 2 hours after wake time (yellow sun symbols) to get phase-advancing light, the “light brake,” to keep their circadian clocks from delaying too far. The filled red triangles represent the Tmin from four different studies,– and show how the circadian clock shifted during the sequence of night shifts and days off. The Tmin reached the target zone (falling within sleep both after night work and on days off) after about a week on the schedule. Abbreviation: DLMO, dim-light melatonin onset.

Figure 2

Sleep times (blue rectangles) and baseline and final temperature minima (Tmin, filled red triangles) for a control subject in study 4. Notes: Control subjects remained in normal room light during night shifts (<50 lux), wore light sunglasses (~36% transmission) when outside during daylight hours, and self-selected their sleep times (from day 2 on). This subject followed a sleep schedule similar to what was required of the experimental subjects. The Tmin for this subject (red triangle on day 14) eventually fell within the times she chose to sleep. Thus she ended up with very little circadian misalignment.Smith MR, Fogg LF, Eastman CI. Practical interventions to promote circadian adaptation to permanent night shift work: study 4. J Biol Rhythms. 2009;24:161–172, Copyright © 2009 by the Journal of Biological Rhythms. Reprinted by Permission of SAGE Publications. Abbreviation: DLMO, dim-light melatonin onset.

Figure 3

Sleep times for a control subject in study 4 that had short daytime sleep after night shifts, sometimes stayed awake for many hours in the morning after night work, napped before some night shifts, and went to sleep and woke up relatively early on days off.Smith MR, Fogg LF, Eastman CI. Practical interventions to promote circadian adaptation to permanent night shift work: study 4. J Biol Rhythms. 2009;24:161–172, Copyright © 2009 by the Journal of Biological Rhythms. Reprinted by Permission of SAGE Publications. Notes: Control subjects did not receive bright light during the night shifts and wore light sunglasses (~36% transmission) when outside. The temperature minimum (Tmin) of this subject did not shift out of the time for night work. Thus, he continued to have circadian misalignment. Abbreviation: DLMO, dim-light melatonin onset.

Figure 4

The number of slow responses on the procedural reaction-time task averaged over three night shifts (days 8–10 in Figure 1) for subjects whose circadian clocks were not realigned (n = 12), partially realigned (n = 21), or completely realigned (n = 6) to night work by the end of their series of night shifts and days off. Notes: The dashed horizontal line shows the average values for all subjects during baseline day shifts. Error bars show the standard error of the mean. The performance of the subjects who were partially or completely realigned was very good compared to the subjects who had the most circadian misalignment. Copyright © 2009. Smith MR, Fogg LF, Eastman CI. A compromise circadian phase position for permanent night work improves mood, fatigue, and performance. Sleep. 2009;32:1481–1489.

Figure 5

The number of lapses (reaction time > 500 milliseconds) at 6:30 am on the simple reaction-time task across successive night shifts for subjects whose circadian clocks were not realigned (n = 12), partially realigned (n = 21), or completely realigned (n = 6) to night work by the end of their series of night shifts and days off. Notes: The dashed vertical line indicates that 2 days off occurred between night shifts 3 and 4. The dashed horizontal line shows the average values for all subjects during baseline day shifts. Error bars show the standard error of the mean. The performance of the subjects who were partially or completely aligned was greatly improved compared to those who had the most circadian misalignment. Copyright © 2009. Smith MR, Fogg LF, Eastman CI. A compromise circadian phase position for permanent night work improves mood, fatigue, and performance. Sleep. 2009;32:1481–1489.

Figure 6

Sleep-and-light schedule designed to reduce the circadian misalignment produced by night-shift work, and thus to improve night-shift performance as well as sleep both after work and on days off. Notes: A permanent night-shift schedule with 5 days of 8-hour night shifts alternating with 2 days off is shown, but the same principles apply for other times and durations of night shifts (eg, 12 hours) and for other sequences of night shifts and days off. Intermittent bright light is used during the beginning of the night shift to help delay the circadian clock. Blue rectangles indicate sleep in the dark. Days off sleep starts late and extends until noon. Sunglasses (G) are used to attenuate phase-advancing light during the commute home. Bright light (outdoor light or from a light box) is used as indicated by the sun symbols as a light brake to keep the rhythms from delaying too far. The expected course of the circadian clock is shown by the triangles representing the temperature minimum (Tmin), the sleepiest time of day. The exact amount of delaying and advancing bright light needed for an individual must be titrated to ensure that the Tmin reaches and remains within the target zone (the time that sleep after work and sleep on days off overlap).