Open questions in sleep and memory research

Sleep is well known to facilitate the consolidation of newly acquired memories. Although this effect has been extensively studied in the past two decades, there are still a number of issues that remain unresolved. In this article, some of the open questions in the field of sleep and memory research are discussed. Particularly, the psychological conditions that determine sleep-dependent memory consolidation as well as possible underlying physiological mechanisms are considered. With regard to the psychological conditions, it will be discussed to which extent the beneficial effect of sleep for memory consolidation depends on the type of learning task, type of retrieval test, relevance of the learning material for the individual, optimum temporal delay between learning and sleep, and amount of sleep. Concerning the physiological mechanisms, this article focuses on the role of different sleep stages and sleep parameters (e.g., sleep spindles, slow oscillations), as well as on the functional importance of memory reactivations (“neuronal replay”) and synaptic downscaling during sleep.