Several studies exist that have conducted research into the effects of different shiftwork patterns on the individual, especially regarding 8 and 12h rosters. The findings of these studies have been largely supportive of longer shifts, however, the effects on work performance are not as clear cut. This study aimed to examine the changeover from an 8h roster to a 12h roster in a power station via monitoring on-shift performance, general health and well-being, sleep and mood behaviour, as well as absence and accident data. Results suggest that the domestic and social life of workers was markedly improved under the 12h roster. Improvements in physical health, sleeping behaviour and mood state of employees were also documented. On-shift performance measures showed an increase in error rates at the end of a 12h shift. Ways of reducing the risk of error towards the end of a 12h shift should be explored. The results of this study suggest that 12h shifts are a valid alternative to 8h shifts in this particular workplace, although tasks that require error-free activities should not be performed towards the end of a 12h shift. Copyright (C) 1999 Elsevier Science B.V.