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...
