The R sleep begins at approx. 90 minutes after sleep onset.1 It reoccurs every 90 to 120 minutes across the night. These results were observed in healthy young adults who sleep well.
The R sleep—paradoxical sleep or active sleep2—on the first cycle extents between 1 and 5 minutes.3 These episodes increase in duration in subsequent sleep cycles.
The R sleep is defined by EGG activation, muscle atonia, sexual arousal4, and bursts of rapid eye movements. Also, stage R is associated with dreaming.
Since 2007, R is used to name REM sleep according to The American Academy of Sleep Medicine (AASM) scoring manual.
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Carskadon, M.A., & Dement, W.C. (2011). Monitoring and staging human sleep. In M.H. Kryger, T. Roth, & W.C. Dement (Eds.), Principles and practice of sleep medicine, 5th edition, (pp 16-26). St. Louis: Elsevier Saunders.3
Freberg, L, (2009). Discovering Biological Psychology, 2nd edition. Wadsworth Publishing.4
Keenan, S., & Hirshkowitz, M. (2011). Monitoring and staging human sleep. In M.H. Kryger, T. Roth, & W.C. Dement (Eds.), Principles and practice of sleep medicine, 5th edition, (pp 1602-1609). St. Louis: Elsevier Saunders.1
Malhotra , R., & Avidan, A. (2014). Sleeping stages and scoring technique. In S. Chokroverty , & R. Thomas (Eds.), Atlas of sleep medicine, 2nd edition, (pp 77-99). St. Louis: Elsevier Saunders. 2