Diversity of REM sleep

Recently, progress has been made in understanding sleep and wakefulness regulation at the molecular and neurocircuitry levels. Research on the evolution of sleep has also advanced remarkably across a wide range of animal species, from invertebrates to vertebrates. Pioneering studies on electroencephalography and other physiological features of sleep have raised the concept of two-stage sleep, categorizing sleep in some animals into two stages: slow-wave sleep (SWS) and rapid eye movement sleep (REMS). Currently, it is widely accepted that SWS and REMS occur in mammals and birds. Although these sleep stages have been observed in a wide range of animals, debates continue regarding their roles and evolutionary origins. This review discusses the diversity of REMS by evaluating their reported similarities and differences across various species. • REMS shares traits across many mammals and birds but still varies significantly among species. • Other vertebrates exhibit a two-stage sleep that closely resembles the SWS and REMS. • Understanding the role and mechanisms of two-stage sleep demands a broad, cross-species approach.