Sleep EEG oscillations and synaptic plasticity in memory consolidation across NREM and REM sleep

Based on a synthesis of recent advances, this review elucidates the distinct yet synergistic roles of non-rapid eye movement (NREM) and rapid eye movement (REM) sleep in memory consolidation. During NREM sleep, cortical slow oscillations (SOs) establish a temporal framework by alternating between UP and DOWN states, facilitating cross-regional communication. SOs phase-couple with thalamic spindles to coordinate hippocampal sharp-wave ripples (SWRs), thereby driving the reactivation and transfer of labile hippocampal memories to neocortical networks for long-term storage. Concurrent synaptic homeostasis optimises network efficiency through global downscaling while selectively strengthening memory-relevant connections. In REM sleep, theta oscillations synchronise limbic-prefrontal-hippocampal circuits, and ponto-geniculo-occipital (PGO) waves further integrate emotional and procedural memories. The cholinergically dominant, monoamine-depleted neurochemical milieu of REM sleep promotes dendritic spine remodelling and sparse neuronal reactivation, enabling affective decoupling and synaptic refinement. Despite progress, key challenges remain in understanding cross-frequency oscillatory coupling among slow oscillations, spindles, ripples and REM-specific rhythms, glial contributions and clinical translation. Future integration of high-density electrophysiology with computational approaches may yield mechanistic models and clinically relevant biomarkers for sleep-based cognitive monitoring and intervention.