Sleep’s contribution to memory formation

The brain state of sleep contributes in a specific way to the formation of long-term memory. Over the past 10 years, research on the psychological and neuronal mechanisms underlying this process has rapidly increased, including studies in humans and rodents across early and late life. Intended to comprehensively cover this research, our review reveals that the majority of findings are consistent with the concept of long-term memory formation during sleep as an active systems consolidation process that concurs with widespread synaptic downselection. In this concept, the repeated neuronal replay of encoded representations, particularly in the hippocampus, in conjunction with brain oscillations hallmarking non-rapid eye movement (non-REM) sleep, provide mechanisms for regulating information flow across brain networks. This interplay drives the consolidation of newly encoded memory into neocortical long-term stores, whereby this neocorticalization of representations goes along with a transformation of memories into more abstract representations. The findings, however, remain controversial as to the nature of memory transformation: What kind of information is eventually consolidated into neocortical networks and how is storage of this information achieved at the synaptic level? Furthermore, the roles of REM sleep in consolidation of, in particular, emotional memory and in shaping representations at the synaptic level are unclear. Future research also needs to elaborate on how consolidation during sleep differs from that during wakefulness, as well as on the changes in sleep-dependent consolidation across the life span. A promising new area arising from this research pertains to brain stimulation techniques developed to enhance memory consolidation during human sleep.