Neuronal learning activity is reactivated during sleep but the dynamics of this reactivation in humans are still poorly understood. Here we use multivariate pattern classification to decode electrical brain activity during sleep and determine what type of images participants had viewed in a preceding learning session. We find significant patterns of learning-related processing during rapid eye movement (REM) and non-REM (NREM) sleep, which are generalizable across subjects. This processing occurs in a cyclic fashion during time windows congruous to critical periods of synaptic plasticity. Its spatial distribution over the scalp and relevant frequencies differ between NREM and REM sleep. Moreover, only the strength of reprocessing in slow-wave sleep influenced later memory performance, speaking for at least two distinct underlying mechanisms between these states. We thus show that memory reprocessing occurs in both NREM and REM sleep in humans and that it pertains to different aspects of the consolidation process.

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Sleep does not preferentially consolidate a specific kind of declarative memory, but consistently promotes overall declarative memory formation. This effect is not mediated by reduced interference.

A fundamental question in memory research is how the brain establishes stable memories while remaining plastic enough to update entries flexibly and to integrate novel information into existing networks. How can the brain protect old memories from forgetting or being overwritten by new learning (French, 1999)? Research on memory stability and plasticity has been a key topic in psychology and neuroscience for more than a century (Lechner, Squire, & Byrne, 1999). At the end of the 19th century, Müller and Pilzecker (1900) realized that memories have to undergo a maturation process to become long-lasting and stable. They noticed that conflicting information intervening between the presentation and recall of word pairs disrupted memory for the to-be-remembered items, and named this phenomenon retroactive interference. Müller and Pilzecker reasoned that a time-dependent and learning-related physiological mechanism must be responsible for the transformation of the labile memory trace into an enduring state, because the vulnerability of a memory trace depended on its age. They termed the process that stabilized memories while increasing their resistance to interference consolidation, and assumed that the progress in memory consolidation can be approximated by how resistant new memories were against interfering information. Around the same time, it emerged that sleep supported the formation of memories. Hermann Ebbinghaus noticed that humans forget less over periods of sleep as compared with wakefulness (Ebbinghaus, 1885). Later experiments found that verbal material was more likely to be remembered when it was learnt shortly