Sleep and memory consolidation

Memory refers to the physiological process in which the brain is able to keep the information obtained from both internal and external sources over an extended period of time. In the brain,neuronal ensembles distributed across multiple regions encode information about past scenes or events and form long-term memories, which then can guide individuals’ behavior in similar scenes or events. Unless been consolidated, these newly-formed memories decay quickly. Sleep is a brain state with multiple physiological functions, including non-rapid eye movement sleep（NREM） and rapid eye movement（REM） sleep. Memory formation involves 3 processes: encoding, consolidation, and retrieval. Both reduced sleep time and fragmented sleep can lead to the impaired consolidation of various types of memory, indicating that one of the important physiological functions of sleep is beneficial for the memory consolidation. In this article, we summarizes the critical evidence that NREM sleep and REM sleep are involved in memory consolidation, and discuss how sleep promotes memory consolidation through the mechanisms such as ensemble reactivation and synaptic modification. Future researches may include further exploration on how sleep coordinates synaptic "down-scaling" with "up-scaling", how REM sleep complements with NREM sleep, the involvement of glial cells activity during sleep in memory consolidation, and whether and how brain regions controlling NREM and REM sleep directly participates in the regulation of memory consolidation.