Delayed Declarative and Procedural Memory Performance Following Interference: A Comparison of a 90-Min Midday Nap, Quiet Rest, and Active Wake

Newly formed memories are vulnerable to interference. Although sleep can stabilize memories and increase resistance to interference, whether quiet rest provides comparable delayed benefits remains unclear. This study compared the effects of a midday nap, quiet rest, and active wake on delayed declarative and procedural memory performance following interference learning. Post-interference performance served as the baseline. Thirty-seven habitual nappers (29 females and 8 males; mean age = 20.21 ± 3.04 years) were randomly assigned to nap (n = 13), quiet rest (n = 12), or active wake (n = 12) conditions, with polysomnography recorded during the assigned condition. Analyses of percentage change from the post-interference baseline revealed no significant group effects or group × time interactions for declarative or procedural memory. Procedural memory accuracy and speed showed greater improvement at next-day noon than in the evening. Exploratory Holm-corrected one-sample Wilcoxon tests showed improved procedural accuracy relative to the post-interference baseline in the nap group in the evening and in the nap and active wake groups at next-day noon, when speed improved in all three groups. These within-group changes do not establish between-group differences. In the nap group, frontal N2 slow spindle density was positively associated with next-day declarative memory accuracy, while slow spindle peak-phase coupling during N2 and N3 sleep was significantly associated with next-day changes in declarative memory accuracy. Greater REM sleep duration and percentage were associated with less improvement in evening procedural memory speed, whereas frontal N3 fast spindle phase–amplitude coupling was negatively associated with evening gains in procedural memory accuracy. No significant EEG–memory associations emerged during quiet rest. Overall, no significant nap-specific or quiet-rest behavioral advantage was detected. The EEG–memory associations were exploratory and do not establish a sleep-specific consolidation mechanism.