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 meta-regressions 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 foralternative sleep-study designs (e.g., 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, multi-causal theoretical account of sleep-induced episodic memory benefits and provide guidance to increase their replicability.
The Sleep Benefit in Episodic Memory: An Integrative Review and a Meta-Analysis