Coupled sleep rhythms for memory consolidation.

Newly formed memories require strengthening and reorganisation (consolidation) after learning. The sleeping brain is hallmarked by slow oscillations (SOs), spindles, and ripples. These rhythms drive neuronal firing rates with increasing efficacy and spatiotemporal precision. While SOs set global windows of excitability and inhibition, nested spindles serve as relays that partially reactivate cortical learning networks, facilitate hippocampal ripples, and bias the content of associated memory reactivation. Ripples activate local memory circuits and drive hippocampal–cortical pattern completion to strengthen and reorganise memory traces. SO–spindle–ripple coupling thus governs consolidation processes at the neuronal (synaptic) and brain-wide (systems) level. How do passing moments turn into lasting memories? Sheltered from external tasks and distractions, sleep constitutes an optimal state for the brain to reprocess and consolidate previous experiences. Recent work suggests that consolidation is governed by the intricate interaction of slow oscillations (SOs), spindles, and ripples – electrophysiological sleep rhythms that orchestrate neuronal processing and communication within and across memory circuits. This review describes how sequential SO–spindle–ripple coupling provides a temporally and spatially fine-tuned mechanism to selectively strengthen target memories across hippocampal and cortical networks. Coupled sleep rhythms might be harnessed not only to enhance overnight memory retention, but also to combat memory decline associated with healthy ageing and neurodegenerative diseases. [ABSTRACT FROM AUTHOR]