The effect of caffeine nap on declarative and procedural memory in elderly

Abstract

INTRODUCTION: Previous studies demonstrated that daytime nap contributed to sleep-dependent memory consolidation, and some studies suggested that nap combined with the use of caffeine have additional benefit on cognitive functioning. However, such effect has not been examined in the elderly population. The present study aimed to examine the effect of nap, combined with the use of different dosages of caffeine, on memory processing in elderly. MATERIALS AND METHODS: The present study recruited twenty-four healthy elderly aged between 61 and 80 years (70.58 ± 5.24 years), who were habitual nappers and non- to moderate- caffeine consumers. We adopted a randomized, double-blind, placebo-controlled within-subject design. Each participant completed 4 experimental sessions, including Wake (W), Nap only (Nap0), Nap with 100 mg caffeine (Nap100), and Nap with 200 mg caffeine (Nap200), with intervals of one-week. Participants were required to stay awake in W condition, and were required to take nap in a 1-h window in the remaining 3 conditions. Placebo or caffeine-containing solutions were administered before rest/nap. Electrophysiological activities were monitored by polysomnography (PSG). A declarative memory task and a procedural memory task were administered before and after the rest/ nap. RESULTS: Two-way repeated measures ANOVA revealed no main effect of condition on declarative or procedural memory. Correlation analysis on the relationship between sleep oscillation (spindle (Sp) density and slow wave activity (SWA) density) and post-nap changes in declarative and procedural memory showed that in Nap0 condition, Sp density was associated with improvement on procedural and declarative memory; and SWA density was associated with improvement on declarative memory. In Nap100 condition, Sp density showed association with improved declarative memory, and SWA density showed association with improvement on both procedural and declarative memory. In Nap200 condition, only SWA density showed correlation with improvement on procedural and declarative memory. CONCLUSION: The benefit of nap on memory consolidation was not found in elderly based on the behavioural measurement. Analysis on sleep oscillation showed that Sp density and SWA density were associated with post-nap improvement on declarative and procedural memory, which was similar to the pattern reported in young adults. Association between Sp and post-nap memory improvement was absent in conditions involved use of caffeine. The underlying mechanism remained to be addressed.