SUMMARY Slow‐wave sleep (SWS) has been theorized to be an intense form of nonREM sleep, but selective deprivation of SWS or Stage 4 sleep has not been shown to cause greater decrements in alertness or performance, compared to deprivation or disruption of the other stages of sleep. The present experiment examined the effects of marked SWS deprivation (SD) for two nights, a control sleep disruption (CD) condition in which minutes of SWS were preserved, and a no sleep disruption (ND) condition. Daytime sleepiness was assessed with the multiple sleep latency test (MSLT) and performance was evaluated with the simulated assembly line task (SALT), neither of which was used in previous studies of SWS or Stage 4 sleep deprivation. In agreement with prior studies, two nights of SD did not cause greater daytime sleepiness than did CD, although sleepiness in both conditions was increased compared to the ND condition. In addition, neither SD nor CD caused declines in performance or mood. However,post hocanalysis suggests an interaction between SWS and sleep duration, such that sufficient SWS may tend to prevent adverse effects of mild sleep loss on waking funct