The current study reports the effect of 0-8-hr naps placed prior to two consecutive nights of total sleep deprivation. A total of 104 young adult male subjects were randomly assigned to one of four prophylactic nap conditions (0, 2, 4 or 8 hr). After a normal baseline night of sleep and a morning of baseline test performance, subjects returned to bed at 1200, 1600 or 1800 hr or not at all prior to a continuous operation that extended until each subject's normal bedtime on the third following night. All subjects who napped arose at 2000 hr, and all subjects maintained the same schedule of computer-administered performance tests throughout the sleep-loss period. Results indicated that performance and alertness in all nap conditions were improved in a dose-response fashion compared to a no-nap control throughout the first 24 hr of sleep loss. However, significant improvement in nap conditions compared to the no-nap condition was not seen in many variables during the second night of sleep loss. Whereas an 8-hr nap prior to an operation maintained performance at a high level for 24-30 hr, significant improvement in alertness and performance as compared to the no-nap control was also documented by shorter naps. No nap could reverse the profound loss of alertness seen during the second night of sleep loss.