Twelve-hour shifts are currently regarded by many workers as one solution to the disruptive effects of shiftwork on health, well-being and lifestyle. Twelve-hour shifts offer larger and more frequent blocks of leisure time than do 8-h shifts. Nevertheless, concern must be addressed about the possible effects of working these additional hours on work quality and productivity and whether they are worked at significant extra cost to the worker. In a study of 75 computer operators, the effect of changing from a predominantly 8 h per shift irregular roster to a 12 h per shift regular roster was investigated. Operators completed a questionnaire covering demographic and health details including the General Health Questionnaire (GHQ), and details about general job satisfaction including the Work Environment Scale (WES). They also completed a 14-day diary of sleeping and eating patterns and mood state at the beginning and end of each shift for the same period. The questionnaires and diaries were completed first under the 8-h shift roster, then again 12 months later after the 12-h shift roster had been worked for 7 months. Work quality, productivity, staff turnover and sickness and other absence data were also collected under the two shift systems. The results showed that changing to the 12-h shift roster produced improvements in health, particularly in psychological health and in reduced feelings of tiredness throughout the work period. The change in working hours was at no cost to feelings of job satisfaction or the worker's perceptions of any particular aspects of the work environment, or to measures of productivity.