This study compared alertness and psychomotor performance over an extended work period in which participants received either a 4-h afternoon nap or four 1-h naps during the night. It was hypothesized that alertness and performance would be superior during the night following the 4-h afternoon nap and that the overall level of performance would be related to the amount of sleep obtained. It was found that when a series of 1-h naps was taken during the normal night period, oral temperature and psychomotor performance also declined. However, performance was relatively improved on the following evening. In contrast, with an effective 4-h prophylactic nap, performance remained near baseline levels across the night. It was concluded that the placement of sleep periods during extended work be based on knowledge of when maximal performance would be required.