Sleep and memory

Sleep plays a crucial role in memory consolidation and has been shown to benefit various types of memory. These beneficial effects, once believed to originate from sleep's protection from external and internal interference, are now thought to stem primarily from reactivation of previously acquired memories during sleep. Sleep-based alterations in memory storage involve temporally synchronized brain waves: hippocampal sharp-wave ripple complexes, thalamocortical sleep spindles, and cortical slow waves. Mechanisms of memory reactivation differ across the classic stages of sleep, such as slow-wave sleep and rapid-eye-movement sleep. The unique contributions of each sleep stage are not fully understood, although slow-wave sleep appears to be particularly critical for the neocortically based consolidation of declarative memories, facilitating the subsequent recall and recognition of facts and events. During sleep, a dynamic interaction between the hippocampus and neocortex can serve to gradually reinforce and transform cortical memory traces. Sleep can thus support stabilization of new memories, integration of new knowledge with existing knowledge, selective strengthening of aspects of some memories, and extraction of gist or discovery of rules within complex collections of memories. This article surveys the leading approaches to studying sleep's role in human memory, and also examines the future potential of sleep-based applications and technologies that might prove useful for treating certain neurological and psychiatric disorders, or for general memory enhancement in healthy populations.