The Sleep Benefit in Episodic Memory: An Integrative Review and a Meta-Analysis

People recall more information after sleep than after an equally long period of wakefulness. This sleep benefit in episodic memory has been documented in almost a century of research. However, an integrative review of hypothesized underlying processes, a comprehensive quantification of the benefit, and a systematic investigation of potential moderators has been missing so far. Here, we address these issues by analyzing 823 effect sizes from 271 independent samples that were reported in 177 articles published between 1967 and 2019. Using multilevel metaregressions with robust variance estimates, we found a moderate overall sleep benefit in episodic memory (g = 0.44). Moderator analyses revealed four important findings: First, the sleep benefit is larger when stimuli are studied multiple times instead of just once. Second, for word materials, the effect size depends on the retrieval procedure: It is largest in free recall, followed by cued recall and recognition tasks. Third, the sleep benefit is stronger in pre-post difference measures of retention than in delayed memory tests. Fourth, sleep benefits are larger for natural sleep and nighttime naps than for alternative sleep study designs (e.g., slow-wave sleep; SWS-deprived sleep, daytime naps). Although there was no obvious evidence for selective reporting, it is a potential threat to the validity of the results. When accounting for selective reporting bias, the overall effect of sleep on episodic memory is reduced but still significant (g = 0.28). We argue that our results support an integrative, multicausal theoretical account of sleep-induced episodic memory benefits and provide guidance to increase their replicability. Public Significance Statement This meta-analysis investigates the effects of sleep versus wakefulness on episodic memory for previously learned information and its context (e.g., location, time). We found a moderate overall sleep benefit that increases (a) when items are studied repeatedly at encoding, (b) when using memory tests for verbal material that lack retrieval cues, (c) when employing pre-post difference measures of retention, and (d) when applying specific sleep study designs such as comparisons of nighttime sleep with daytime wakefulness. Overall, these results suggest that sleep strengthens both storage and retrieval of episodic memories.

Automated summary

Sleep has a moderate benefit on episodic memory, especially when stimuli are studied repeatedly, when using memory tests for verbal material without retrieval cues, and when employing pre-post difference measures of retention. The benefits are larger for natural sleep and nighttime naps.