Sleep-stage dependent patterning of slowly propagating brain activity.

Endogenous brain activity plays a key role during sleep, facilitating important functions like learning and memory consolidation. Its precise mechanisms are not yet understood. Recent functional MRI (fMRI) research has revealed structured multi-second waves that travel across the cortex along a hierarchical functional gradient during wakefulness. However, how these waves are modulated across different sleep stages remains unclear. Here, we present compelling evidence of significant changes in both the frequency and spatiotemporal patterns of these waves across sleep stages. Notably, during rapid eye movement (REM) sleep, we observed a predominance of propagations from sensory/motor areas to higher-order networks. These patterns were distinct from other sleep stages, with key regions like the thalamus, pons, and visual cortex-linked to PGO waves-activating at the earliest phase of waves. Similar to PGO waves, these fMRI waves were coupled with REM activity, suggesting their potential role in supporting memory consolidation and learning processes.