An understanding of the consequences of sustained operations involving prolonged sleep deprivation is important to the military. Losses in cognitive performance of the order of 30% after one night and 60% after two nights of sleep loss have been shown to occur in several studies. Napping strategies have been proposed as one coping strategy for these performance decrements. An alternate solution is the use of stimulants. Modafinil may offer a safer alternative to more commonly used psychostimulants. It has been shown to counteract the effects of sleep deprivation with fewer side effects than amphetamine. Recent studies on the effects of modafinil during sleep deprivation are reviewed and compared with those of other stimulants such as amphetamine and caffeine, and to prophylactic naps. Recommendations are proposed for the use of modafinil in sustained military missions. For missions of about 24 h, modafinil is preferable to naps. For longer missions, naps should be considered, along with the concomitant use of modafinil to help maintain performance levels. The authors discuss apparent 'overconfidence' and hyperthermia-inducing effects of modafinil, and advise that these effects be taken into account if modafinil is to be used in any mission conducted in the field or in hot environments. It is also recommended that individual sensitivity to the drug should be tested before any mission of a sustained nature, especially if it involves small operational groups or combat aircrew, where the impact of such effects on individuals would be more critical.