Introduction
Learning, memory and the neural plasticity underlying these processes are fundamental characteristics of animals and humans that allow for adaptation in changing environments. Research over the past years has shown that sleep after learning facilitates the quantitative strengthening of newly encoded and initially instable memory traces (for review, please refer to [1]). Quantitative strengthening refers to the veridical preservation of stored information and stabilises memories against disruptive interference and decay [2].
More recently, it has been proposed that sleep might also promote the qualitative reorganisation of memories; i.e., the emergence of new memory content that has not been directly learned. This reorganisation of memories during sleep will be the focus of the current review.
A comprehensive model of sleep and memory was recently proposed [3]. This model overarches the entire fate of a novel memory trace from selectivity in initial processing to its subsequent strengthening and further memory evolution; these processes may, at least partially, be interlinked and complementary. The present review elaborates on memory evolution in the form of qualitative changes in memory content rather than the earlier steps of initial processing or strengthening.
Specifically, we review and integrate the current literature related to the two major memory systems, the declarative and the procedural motor systems, from a behavioural perspective. Subsequently, we review the potential neural mechanisms underlying these systems and the modulation of memory reorganisation by emotions. In the final translational section, the potential implications of sleep-related memory reorganisation for the aetiology and treatment of mental disorders are discussed.