The advice of contemporary science-based practitioners of sleep and fatigue management is to return shift-workers to being awake during the day and asleep at night as often, quickly, and for as long possible, to keep circadian rhythms anchored to the light-dark cycle of the sun. This is in stark contrast to the recommendation we used to give which was to keep workers on the same shifts for as long as possible if they needed to be awake during any portion of the night. The hope used to be that their circadian rhythms would eventually adapt to the required sleep-wake patterns, and fatigue risk at night would be minimized. Research eventually proved the old recommendation was wrong for at least three reasons:
- Workers were rarely on the same shift long enough to allow all circadian rhythms to synchronize to the new sleep-wake patterns. For example, many of the shift-worker's circadian rhythms did not fully synchronize to being awake at night after months of straight night shifts.
- Even with advanced training on circadian rhythm adjustment, it was too difficult to expertly manage all biological time cues and avoid inappropriate exposure to them. For example, in the summer months, night shift workers were exposed to bright morning sun on their commutes home, and shift-workers usually reverted to being awake during the day on their off-duty days.
- Repeatedly manipulating circadian rhythms over long periods of time, like a career in shift-work, increased significant health risks. The small reduction in fatigue risk achieved by synchronizing some circadian rhythms to being awake at night was not worth the potential negative health outcomes.
There are situations where contemporary anchoring advice may not apply, and it might make sense for shift-workers to manipulate biological time cues to adjust to new sleep-wake patterns. The first situation is when workers change time zones for prolonged periods. If the work schedule at their new location allows them to be awake when the sun is up and asleep after it sets, exposure to biological time cues will occur at clock times that naturally adjust circadian rhythms to the new sleep-wake patterns. In this situation, it would make sense for your shift-workers to work with the natural adjustment, rather than try to keep circadian rhythms anchored to the previous time zone.
A second scenario is when workers travel far enough north or south to locations where close to perpetual night or day occurs and remain there for prolonged periods. While artificial light can help keep circadian rhythms anchored to the light-dark cycles shift-workers were originally anchored to, complete darkness and daylight make it difficult to do so. When it is completely dark, shift-workers’ brains will receive night signals during the day, and when it is completely light, their brains will receive day signals during the night, both of which will cause some degree of circadian rhythm desynchronization.
In this scenario, shift-workers should manipulate all biological time cues, and especially light, to the best of their abilities to anchor their circadian rhythms according to their desired sleep-wake pattern. For example, if a worker needs to be awake during night hours when complete daylight is occurring, they should seek exposure to sunlight during those hours and avoid sunlight during the daytime hours (the desired sleep time). Whereas if the worker needs to be awake during the day under the same light condition, sunlight should be avoided during the nighttime hours (the desired sleep time). Alternatively, during complete darkness, workers should seek exposure to full spectrum artificial light during their desired wake period and avoid light during their desired sleep periods. In both light conditions, all other biological time cues should also be manipulated to occur at appropriate times during the worker’s desired sleep and wake periods.
Although it may seem like a complicated twist, when workers follow rotational shift patterns in these north and south locations, contemporary anchoring advice does apply. Workers should anchor their circadian rhythms to a stable day-night pattern paying particular attention to the timing of light exposure. Full spectrum light exposure should be sought during the day and avoided at night.
In prolonged time zone change and north-south scenarios where new sleep-wake patterns may be desired, your shift-workers may be able to more efficiently adjust their circadian rhythms by being careful with exposure to the time cues, but they will not be able to speed up adjustment beyond the capabilities of human biology. It usually takes a minimum of 2/3 of a day of exposure to biological time cues at appropriate times per westward time zone or clockwise sleep period change, and one day per eastward time zone or sleep period change to synchronize most circadian rhythms to the new time zone or sleep period. For example, if your shift-workers travel six time zones west, it will take them four days to adjust. When they return east, it will take six days. Adjustment westward or clockwise occurs, in part, more quickly because it is biologically easier for the circadian rhythm of sleep to phase delay. When we travel west, we stay awake longer to synchronize sleep onset with the appropriate local clock time. This increases our sleep drive, which makes it easier to fall asleep, resetting our bedtime and facilitating adjustment of many other circadian rhythms. In addition, some circadian rhythms are slightly longer than 24 hours in young to middle-aged adults which naturally predisposes them to staying up later and falling asleep closer to the bedtime in the new time zone.
When we travel east and try to fall asleep at the appropriate clock time, our sleep drive is low at the desired sleep time. The slightly longer than 24 hour circadian rhythms introduce another difficulty because they will exert an alerting influence at the desired sleep time. Both influences make it biologically challenging to fall asleep earlier than normal which limits how well the new sleep period facilitates the adjustment of other circadian rhythms. In addition, some circadian rhythms will only resynchronize in a phase delayed/clockwise fashion; they are incapable of moving backwards in a phase advanced fashion when we travel east and reversing their direction of synchronization when we travel west. This further contributes to the slower rate of eastward circadian rhythm adjustment.
The relative ease of westward/clockwise compared to eastward/counterclockwise biological change due to natural circadian rhythm phase delays can also be used in shift schedule design. For shift-workers remaining in the same time zone and working shifts that make it difficult to obtain a full sleep period during the night hours, the contemporary advice is to rotate schedules clockwise if rotations are needed.