It is generally agreed that workday durations of 12h or more are not advisable where workload levels are very high. However, there is no clear research basis for the identification of workload levels which should preclude the introduction of extended shifts. In the present study, the effects of varying workload levels and shift durations were investigated both in the laboratory and in a workplace. In the laboratory study subjects performed either cognitive work (two workload levels) or physically demanding work (heart rate maintained at 35beats/min above resting), and there were significant effects of both shift duration and workload level on various performance tests, bodily discomfort, hand steadiness and alertness. In the physically demanding condition, perceived workload was higher on 12h days despite heart rate being unchanged. In a field study, 34 workers participating for three days each, half of them on 8h days and half on 12-h days; workload data were obtained for each subject at each half-day data collection point. The combination of higher workload and 12-h workdays produced greater fatigue, as manifested in increased bodily discomfort, decreased alertness, decreased hand steadiness and increased errors on the grammatical reasoning task. In addition to these interacting effects of workload and work duration, there were main effects of both workload and work duration on bodily discomfort, and of workload on hand steadiness. Relevance to industry The results have relevance for managers and employees regarding the adoption of extended shift systems, particularly in relation to the nature of job demands, overall workload levels, and the criticality of human performance for system safety and efficiency.