A questionnaire was performed in order to investigate the effect of naps during duty on recovery from fatigue. Studies were performed on 12 male computer operators in each of two chemical plants working a four-team three-shift system. In one of the plants, the shift workers were able to take a 2 h nap during the night shift (nap group). In the other plant, no nap was taken (no-nap group). Before and after the two consecutive night shifts, both the nap and no-nap groups greatly extended their night sleeps, but the daytime sleep taken by the no-nap group during this period was significantly longer than that of the nap group on both the first and second days. No significant difference was found when comparing the length of the day sleep of the no-nap group with the total sleeping time (night-time nap plus subsequent day sleep) of the nap group. Therefore a night-time nap enables part of the essential sleep to be taken in advance of the day sleep following night work. During night work, both tbe nap and no-nap groups exhibited an increase in the sleepiness scores and also in the subjective feelings of fatigue concerning the complaints related to drowsiness, dullness and difficulty in concentration. However, it was found that for the no-nap group these effects continued for a large part of the recovery period following night work. It can therefore be surmised that naps taken during night-time work can be to a certain extent aid recovery from the fatigue caused by that work.