Based on these findings, the active system consolidation theory postulates that the covert and repeated reactivations of newly encoded memories during NREM sleep sub-serve these system consolidation processes of reorganization and integration (Diekelmann & Born, 2010; Stickgold & Walker, 2013). System consolidation refers to the two-stage model of memory consolidation (Marr, 1971; McClelland, McNaughton, & O’Reilly, 1995), stating that initial encoding of memories critically relies on a fast learning memory system (i.e., the hippocampus for declarative memories). After encoding, the information is gradually redistributed into a second, slow-learning memory system in cortical brain areas and integrated into pre-existing knowledge networks. Thereby, memory representations gradually loose their retrieval-dependency on hippocampal structures. According to the active system consolidation theory, this process of gradual system consolidation is actively supported by sleep based on repeated memory reactivations interleaved with reactivation of associated older memories (Rasch &Born, 2007)