Altered sleep/wake patterns involve, by definition, displaced sleep. The present review concludes that mental performance is strongly influenced by many forms of displaced sleep. Being exposed to the circadian low (during work/activity), extended time awake or reduced duration of sleep will impair performance. The effect is most pronounced in the laboratory setting, however, even if a number of studies have shown effects of for example night work on neuropsychological tests, and simulated work. In real shift work situations performance changes have been less pronounced. No studies have evaluated the effects on production, but accidents and serious mistakes have been clearly established in road transport and there seems to be clear effects also in health care. The effects are similar in connection with flights across several time zones (jet lag) but less data are available. It is suggested that there is a need for establishing the significance of impaired performance due to work hours in white collar and service work. Also the notion of individual differences in performance impairment is an important issue.