Slow Frontal Spindle Activity and Declarative Memory Retention During Daytime Naps

Memory consolidation refers to how newly encoded memories are strengthened and retained over time, and ample evidence indicates that sleep supports this process for both procedural and declarative memories. Although sleep spindles during non-rapid eye movement (NREM) sleep have been associated with consolidation, it remains unclear whether all spindle types contribute equally. Spindles vary in frequency and topography, slow spindles (≤ 12.5 Hz) predominating frontally, fast spindles (> 12.5 Hz) parietally and recent work suggests that procedural memory consolidation relates to the temporal organization of spindles in 'trains' (events occurring < 6 s apart). We investigated whether a similar mechanism operates for declarative memory during daytime naps. Participants (Nap, N = 23; No-Nap, N = 15) completed an object-location task involving 36 item-location associations, tested immediately after learning and again following a 90-min nap opportunity or an equivalent wake period. The Nap group exhibited significantly better delayed memory, measured by combined recall-associative recognition scores and a greater proportion of nappers maintained or improved performance. Within the Nap group, memory was positively associated with local spindle density at frontal and parietal sites and with the proportion of slow frontal spindles clustered in trains during NREM2. Contrary to our expectations, however, these associations did not reach significance under two-tailed testing, possibly reflecting the limited sample size and thus lack of statistical power for an experimental design based on naps. Together, our results demonstrate a robust behavioural benefit of daytime naps for declarative memory retention and, more tentatively, point to slow frontal spindle activity as a candidate correlate for future work.