The 70 years since the discovery of rapid eye movement sleep: history, electroencephalographic features and unsolved issues.

This article aims at providing a comprehensive review of the historical discovery and following research on rapid eye movement (REM) sleep, highlighting its manifold nature as a behavioural, electrophysiological and dreaming state. Pioneering works conducted by Aserinsky, Kleitman, Dement and Jouvet established the foundational understanding of REM sleep recurrence, brainstem mechanisms, and the paradoxical coexistence of electroencephalographic activation and muscle atonia. We focus on REM sleep homeostasis, emphasising its role in emotional recovery and the consequences of REM deprivation, such as the REM rebound effect. We also analyse the periodicity of REM sleep, its ultradian rhythm, and the physiological mechanisms underlying its regulation. Additionally, the article discusses the entangled relationship between arousals, sleep, and consciousness, pointing out the distinction between non-REM and REM sleep-related arousals, and the similarities between REM sleep and wakefulness.