The effect of daytime naps on the recall of verbal material

Although recent research showed the positive impact of daytime naps on procedural memory, their effects on declarative memory have been so far almost completely neglected. The aim of the present study is to investigate whether a daytime nap can have a facilitating effect on the recall of verbal material and, if so, which sleep characteristics play a role. Our data show that performance at the declarative task was globally better after a two-hours nap than after the same retention interval spent awake. This resulted from a maintenance of the memory scores over three successive recall tests (14, 16, 19h) in the former condition, compared to the worsening found in the latter. On one side, this might be partly dependent on the beneficial effect of naps on alertness as shown by the subjective sleepiness scores at the three tests: however, differences observed between naps with and without REM sleep suggest that there is an influence of diurnal sleep episodes on memory consolidation which could be related to the presence of both sleep states.