Daytime napping: Effects on human direct associative and relational memory

Sleep facilitates declarative memory processing. However, we know little about whether sleep plays a role in the processing of a fundamental feature of declarative memory, relational memory – the flexible representation of items not directly learned prior to sleep. Thirty-one healthy participants first learned at 12 pm two sets of face–object photograph pairs (direct associative memory), in which the objects in each pair were common to both lists, but paired with two different faces. Participants either were given approximately 90 min to have a NREM-only daytime nap (n = 14) or an equivalent waking period (n = 17). At 4:30 pm, participants who napped demonstrated significantly better retention of direct associative memory, as well as better performance on a surprise task assessing their relational memory, in which participants had to associate the two faces previously paired with the same object during learning. Particularly noteworthy, relational memory performance was correlated with the amount of NREM sleep during the nap, with only slow-wave sleep predicting relational memory performance. Sleep stage data did not correlate with direct associative memory retention. These results suggest an active role for sleep in facilitating multiple processes that are not limited to the mere strengthening of rote memories, but also the binding of items that were not directly learned together, reorganizing them for flexible use at a later time.