STUDY OBJECTIVES: In this study we examined the benefit of a daytime nap containing only NREM sleep on the performance of three declarative memory tasks: unrelated paired associates, maze learning, and the Rey-Osterrieth complex figure. Additionally, we explored the impact of factors related to task acquisition on sleep-related memory processing. To this end, we examined whether testing of paired associates during training leads to sleep-related enhancement of memory compared to simply learning the word pairs without test. We also examined whether strength of task acquisition modulates sleep-related processing for each of the three tasks. SUBJECTS AND PROCEDURE: Subjects (11 male, 22 female) arrived at 11:30, were trained on each of the declarative memory tasks at 12:15, and at 13:00 either took a nap or remained awake in the sleep lab. After the nap period, all subjects remained in the lab until retest at 16:00. RESULTS: Compared to subjects who stayed awake during the training-retest interval, subjects who took a NREM nap demonstrated enhanced performance for word pairs that were tested during training, but not for untested word pairs. For each of the three declarative memory tasks, we observed a sleep-dependent performance benefit only for subjects that most strongly acquired the tasks during the training session. CONCLUSIONS: NREM sleep obtained during a daytime nap benefits declarative memory performance, with these benefits being intimately tied to how well subjects acquire the tasks and the way in which the information is acquired.