Academic journal article Scandinavian Journal of Work, Environment & Health

A Systematic Review of the Sleep, Sleepiness, and Performance Implications of Limited Wake Shift Work Schedules

Academic journal article Scandinavian Journal of Work, Environment & Health

A Systematic Review of the Sleep, Sleepiness, and Performance Implications of Limited Wake Shift Work Schedules

Article excerpt

Many industries require continuous, around-the-clock operations, for example, defense, emergency services, health, manufacturing, mining and transport, as well as more rarefied occupations such as space operations (1-6). These industries face a challenge in how to prevent the decrements in safety, alertness, and performance that arise from spending a prolonged time at work and working nights. One strategy has been to redistribute time-at-work into smaller aliquots that are spaced equally across the 24-hour period. Commonly employed short cycling work/rest routines or limited wake shift work (LWSW) schedules include 6 hours-on/6 hours-off, 4 hours-on/8 hours-off and 8 hours-on/8 hours-off schedules. These types of shift schedules minimize the number of consecutive hours at work, promote rest opportunities between work bouts, and allow for at least some opportunity for sleep during the night for all workers (7-11). Given the safety-critical nature of many of the industries that employ LWSW and the emerging research activity in this field, a review of the current literature is timely. In particular, there is a need to identify which LWSW schedules best promote sleep, minimize sleepiness, and facilitate performance.

There is a general consensus that the impact of shiftwork on sleep, sleepiness, and performance is mediated by three factors: (i) time-of-day or circadian effects; (ii) the length of time-at-work (and conversely the opportunity for sleep); and (iii) the consistency of start and finish times (8, 12, 13). Taking each of these factors in turn, the impact of the circadian system is such that the maximum drive for sleep, the highest levels of sleepiness, and lowest performance tend to occur during the early hours of the morning (12, 14) also known as the circadian nadir (15). By contrast, the likelihood of sleep and, likewise, sleepiness are at their lowest in the early evening period also known as the "forbidden sleep zone" or the "wake maintenance zone" (WMZ) (16). Sleep, sleepiness, and performance are also strongly influenced by the length of time spent awake and, in turn, the opportunity for sleep, which are primarily dictated by shift length and the timing of shift cycles (8). Generally, shift schedules that possess a high work-torest ratio (ie, where workers spend a longer time at work and have shorter times between shift) are associated with poorer circadian alignment and shorter sleep (17). Likewise shift schedules where start and finish times are inconsistent from one 24-hour cycle to the next and, as a consequence, the timing work/rest bouts (18, 19).

While the factors identified above are relatively well explored in more commonly worked shiftwork schedules (eg, 8 hours-on/16 hours-off), they have received less attention in LWSW. Nevertheless, it is expected that the same trends will be evident. Specifically, it is hypothesized that LWSW schedules will be associated with shorter sleep, greater sleepiness and poorer performance where (i) more of the work time falls across the circadian nadir, (ii) more of the sleep opportunity falls during the WMZ, (iii) the work-to-rest ratio is high, and (iv) the consistency in the timing of work/rest bouts from one 24-hour cycle to the next is low (20, 21). The aim of this review is to test these hypotheses and determine which LWSW schedules best promote sleep, alertness, and performance. Given the differences across studies in schedules and methodology, where possible, these hypotheses were tested using meta-analyses (regression modelling). Where available study measures render analysis unfeasible, narrative review of studies were conducted.


LWSW definition

A significant challenge in accessing LWSW literature is the lack of a universally accepted definition, or indeed terminology, to describe roster schedules that employ short sleep/wake rotations. Industry-specific conventions include "watchstanding" (marine), "relay" (rail), "twoup" (rail and road), and "platoon" (emergency services). …

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