Twenty-four-hour operations are a universal feature of many organisations, such as transportation, manufacturing, production, health care and the military. Taylor et al. (1997) state that approximately 20-25% of employees in the manufacturing industry are shift workers. There are individual differences in the ability of people to adapt to shift work. People with a history of sleep disorders, who are over 50 years of age, have a history of gastrointestinal complaints, or a preference for rising early, are associated with difficulties in adjusting to night work (Monk, 1990). The ability to sleep at unusual times and overcome drowsiness are associated with shift work tolerance (Costa et al., 1989). Knauth and Hornberger (2003) identify a number of measures that can be taken to optimise the well-being of shift workers and mitigate the effects of shift work on performance and alertness. Each of these preventative and compensatory measures for shift workers is discussed below.

Ergonomic shift system design

There is no ideal shift system. No system is without drawbacks, so it comes down to selecting the ‘least worst’ shift system. It is possible to make a number of recommendations regarding the direction and speed of shift rotation, shift duration, when shift changeover should occur, and time off between shifts:

Direction and speed of shift rotation. A forward rotation (morning to evening to night) produces less fatigue than a reverse rotation (night to evening to morning). As described above, the internal push from the circadian cycle to run slightly longer than 24 hours would favour the forward rotation.

Speed of rotation can range from none (fixed shifts), to very slow (three to four weeks), to rapid (every two to three days). The worst possible schedule is to work between four and seven nights and then switch to day shift. This results in the circadian rhythm being disrupted, and then beginning to adapt just prior to being disrupted again. Having a permanent night shift team may seem to be the ideal solution by having people who are fully adapted to working at night. However, working a permanent night shift may be detrimental to the mental well-being of night-shift workers as it minimises the contact with friends and family, and only those individuals who are ‘night owls’ are likely to fully adapt. Spencer et al. (2006) reviewed six studies that examined adjustment of the circadian rhythm of permanent night-shift workers. They concluded that only 7% of workers showed evidence of ‘good’ adjustment. However, 29% of workers showed evidence of at least some adjustment that would benefit performance on night shift.

Wedderburn (1991) proposes that the ideal schedule is to work as few night shifts as possible in a row (ideally one) so that the disruption to the circadian rhythm is minimised. However, a slow rotation has the advantage of allowing a worker’s circadian rhythm to adapt to the schedule. Whether a fast or slow rotation is best depends on the type of work being carried out. A fast rotation means that a night shift is staffed by unadapted individuals who are suffering from partial sleep loss. Therefore, in this situation it would be desirable to avoid high-risk, demanding tasks during the night shift. Alternatively, if a high work tempo during the night shift cannot be avoided, a slow shift rotation may be more desirable in terms of safety and productivity.

Time of shift changeover. Twenty-four-hour operations are usually divided into two or three shifts. In a three-shift structure, day shift starts around 5am-8am, and ends around 2pm-6pm. Evening shift starts around 2pm-6pm, and ends around 10pm-2am. Night shift starts around 10pm-2am and ends around 5am-8am (Rosa and Colligan, 1997). Knauth and Hornberger (2003) recommends that the morning shift should not start too early (6am is better than 5am) to maximise hours of sleep. The evening shift should not end too

late (10pm is better than 11pm) and should end earlier at the weekend to increase social contact and maximise hours of sleep. Finally, the night shift should end as early as possible to increase the hours of sleep during nighttime. It can be seen that it is impossible to improve the timing of one shift without having a negative effect on the other shifts.

  • Time off between shifts. Knauth and Hornberger (2003) recommend a maximum of five to seven days of work, with adequate time off (at least 11 hours) between shifts. Research has found that the shorter the time between shifts, the less time people sleep (Kurumatani et al., 1994). fatigue has also been shown to be a factor in accidents in the offshore oil industry due to the long periods of work (14-21 days). Miles (1999) describes research carried out with offshore accident rates by Connolly (1997) in which the incidence of serious injury in comparison to all injuries was found to increase with increasing days working offshore time. Knauth (1995) also outlined several studies that showed an increase in accidents and errors over consecutive night shifts. Spencer et al. (2006) examined seven studies in which accidents and injuries were reported over at least four successive night shifts. They found that on average, when compared with the first night, the risk of injury was 6% higher on the second night, 17% higher on the third night, and 36% higher on the fourth night.
  • Shift duration. There is no strong evidence on the length of shift that is the best in terms of safety and productivity. Knauth and Hornberger (2003) state that many studies have been published comparing eight-hour with nine-hour, or 10-hour with 12+ hours shift lengths, with contradictory results. Dembe et al. (2005) found that working at least 12 hours per day was associated with a 37% increased hazard rate and working at least 60 hours per week was associated with a 23% increased hazard rate. Tucker et al. (1996) compared two groups of chemical workers: one group working a 12-hour shift, and one group working an eight-hour shift. The two groups did not differ on most outcome measures, although the differences that did exist suggested advantages for the 12-hour shift workers over the eight-hour shift workers, except for levels of alertness at certain times of day. Thus, it would appear that the other factors associated with shift design are of greater importance than the duration of the shift.

A summary of recommendations for ergonomic shift design is shown in Table 8.4.

Table 8.4 Summary of ergonomic shift design recommendations

Direction of rotation

Forward rotation

Speed of rotation

Either fast, or slow, dependent upon the type of activity.

time of shift change

Morning shift - should not start too early. Evening shift - should not end too late. Night shift - should end as early as possible.

time off between shifts

A maximum of 5-7 days continuous work, with at least 11 hours between shifts.

Shift duration

No clear recommendations.

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