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Circadian rhythms in performance exist for a wide range of tasks studied under laboratory conditions; they also occur in measures of efficiency obtained in real-life situations. The rhythms appear to be related to the daily cycle of sleep "need", and the largest negative variations in performance are normally observed when this need is greatest. The detailed forms of the circadian functions are influenced to some extent by individual differences in age and personality. However, task factors are of equal, if not greater, importance in determining the nature of a performance rhythm, since its basic phase is markedly different in cases involving memory. This latter finding raises the practical problem of job design in relation to time of day, and a taxonomic approach is suggested as the most fruitful way of arriving at the optimal solution for particular work situations requiring different abilities.

A survey of the different types of shift-work systems in use, and the incidence of shift work in different industries and countries, is followed by a discussion of (a) the effects of shift work on health and (b) the physiological problems raised by the phase-shifting of the circadian cycle in night workers. Summaries of the existing knowledge of the effects of shift work on performance efficiency, accidents, and family and social life are then given, and a set of criteria for designing optimal shift systems is proposed. Next, the questions of selection for shift work and the provision of health services for shiftworkers are discussed. Finally, the need for further research on the problems of shift work is explained, and suggestions are offered on the lines such research should follow.