Objectives-Shiftwork is now a major feature of working life across a broad range of industries. The features of the shift systems operated can impact on the wellbeing, performance, and sleep of shiftworkers. This paper reviews the current state of knowledge on one major characteristic of shift rotas-namely, shift duration. Evidence comparing the relative eVects of eight hour and 12 hour shifts on fatigue and job performance, safety, sleep, and physical and psychological health are considered. At the organisational level, factors such as the mode of system implementation, attitudes towards shift rotas, sickness absence and turnover, overtime, and moonlighting are discussed. Methods-Manual and electronic searches of the shiftwork research literature were conducted to obtain information on comparisons between eight hour and 12 hour shifts. Results-The research findings are largely equivocal. The bulk of the evidence suggests few diVerences between eight and 12 hour shifts in the way they aVect people. There may even be advantages to 12 hour shifts in terms of lower stress levels, better physical and psychological wellbeing, improved durations and quality of oV duty sleep as well as improvements in family relations. On the negative side, the main concerns are fatigue and safety. It is noted that a 12 hour shift does not equate with being active for only 12 hours. Conclusions-There can be considerable extension of the person's time awake either side of the shift. However, the eVects of longer term exposure to extended work days have been relatively uncharted in any systematic way. Longitudinal comparative research into the chronic impact of the compressed working week is needed. (Occup Environ Med 1998;55:217-229) Keywords: shift duration; health; sleep; 12 hour shifts In response to economic, technological, and social pressures a 24 hour organisation of the workplace is now common. This requires the workforce to accept and become adapted to many diVerent forms of shiftwork schedule. However, the adverse eVects of shiftworking are well chronicled.1-3 They include biological disruption to physiological processes, including the sleep-wake cycle 4 ; the impairment of physical health and psychological wellbeing 3 5 ; problems with alertness, performance, and safety, 6 7 and lastly, interference with social and domestic life. 8 9 The extent to which shiftwork aVects the individual person depends largely on the job being done, characteristics of the individual, the organisational and social environments, and features of the shift system. This potential impairment to functioning exists because daily rhythms in human physiology, hormone concentrations, biochemistry, and behaviour have become entrained to the most reliable and predictable cyclic changes in the physical and social environments. Under shiftworking conditions, and especially when required to work at night, the components of this system of circadian rhythms are regularly disrupted as a result of having to alter the activity-rest cycle.Many thousands of...

The authors developed and tested a process model of adaptation to shiftwork, which hypothesizes that various individual and situational variables influence the development of sleep and social and domestic disturbances. Both types of disturbances trigger various types of coping behavior, leading to several proximal outcomes. The end result is the development of chronic health problems in the form of digestive and cardiovascular symptoms. The model was tested with survey data collected from 2 samples of nurses (N = 1,532) in the United Kingdom and was cross-validated against a 3rd sample of industrial workers (N = 370). Results indicate support for the model across the 3 samples, although some sample-specific and subgroup effects were found. Results have direct implications for the development of shiftwork theory and interventions.

Objectives-Previous research has produced conflicting evidence on the relative merits of advancing and delaying shift systems. The current study assessed the eVects of the direction of shift rotation within 8 hour systems, upon a range of measures including sleep, on shift alertness, physical health, and psychological wellbeing. Methods-An abridged version of the standard shiftwork index which included retrospective alertness ratings was completed by four groups of industrial shiftworkers on relatively rapidly rotating 8 hour systems (n=611). Two groups worked continuous systems that were either advancing or delaying; the other two groups worked discontinuous systems that were either advancing or delaying. Results-Few eVects were found of direction of rotation on chronic measures of health and wellbeing, even when the systems incorporated "quick returns" (a break of only 8 hours when changing from one shift to another). This was despite the use of measures previously shown to be sensitive to the eVects of a broad range of features of shift systems. However, advancing continuous systems seemed to be associated with marginally steeper declines in alertness across the shift (F (3,1080)=2.87, p<0.05). They were also associated with shorter sleeps between morning shifts (F (1,404)=4.01, p<0.05), but longer sleeps between afternoons (F (1,424)=4.16, p<0.05). Conclusions-The absence of negative eVects of advancing shifts upon the chronic outcome measures accorded with previous evidence that advancing shifts may not be as harmful as early research indicated. However, this interpretation is tempered by the possibility that diYcult shift systems self select those workers most able to cope with their deleterious eVects. The presence of quick returns in advancing continuous systems seemed to impact upon some of the acute measures such as duration of sleep, although the associated eVects on alertness seemed to be marginal.

The effects on industrial shift workers of the timing of the changeover from the night to morning shift and of the length of the shift were examined. Two groups changed over at 6 a.m. and worked either 8-hr or 12-hr shift systems; the other 2 groups changed over at 7 a.m., working either 8- or 12-hr systems. Night sleeps between consecutive shifts that started at 6 a.m. were shorter and more disrupted than those starting at 7 a.m. Day sleeps following night shifts that finished at 6 a.m. were longer and less disrupted than those finishing at 7 a.m. Early starts were associated with poorer psychological and physical health. These effects of changeover time are considered in relation to the circadian rhythms in sleep duration and propensity. Although several measures favored 12-hr shifts, physical health indicators appeared to favor 8-hr systems, especially in combination with late changeovers. However, the observed effects of shift length on chronic outcome measures are somewhat inconsistent with previous research findings.