Cracking the complexity of REM sleep

Rapid eye movement (REM) sleep is a unique state classically defined by brain activation and muscle paralysis. It is now recognized as a dynamic process involving coordinated oscillations and phasic behaviors, with substantial variations across development and species. Neural circuit studies have identified ever-expanding brain networks that regulate state generation and individual components of REM sleep. To account for this structured yet flexible nature, we propose a hierarchical circuit framework in which core REM sleep features are generated by brainstem nuclei, and adaptively tuned by hypothalamic, cortical, and neuromodulatory systems. From this component- and circuit-based perspective, we synthesize recent advances showing how theta oscillations and distributed forebrain circuits mediate REM sleep functions in memory, emotion, and cognition. We finally outline future research paths towards a more refined and integrative understanding of REM sleep.