Editing Sleep (section)

===Non-REM and REM sleep===
Sleep is divided into two broad types: [[non-rapid eye movement sleep|non-rapid eye movement]] (non-REM or NREM) sleep and [[rapid eye movement sleep|rapid eye movement]] (REM) sleep. Non-REM and REM sleep are so different that physiologists identify them as distinct behavioral states. Non-REM sleep occurs first and after a transitional period is called [[slow-wave sleep]] or deep sleep. During this phase, body temperature and heart rate fall, and the brain uses less energy.<ref name=MaquetEtAl2006 /> REM sleep, also known as paradoxical sleep, represents a smaller portion of total sleep time. It is the main occasion for dreams (or [[nightmare]]s), and is associated with desynchronized and fast brain waves, eye movements, loss of muscle tone,<ref name="ninds">{{cite web |title=Brain Basics: Understanding Sleep |url=https://www.ninds.nih.gov/Disorders/Patient-Caregiver-Education/Understanding-sleep |website=National Institute of Neurological Disorders and Stroke}}</ref> and suspension of [[homeostasis]].<ref>{{cite book | veditors = Parmeggiani PL | date = 2011 | title = Systemic Homeostasis and Poikilostasis in Sleep: Is REM Sleep a Physiological Paradox? | location = London | publisher = Imperial College Press | isbn = 978-1-94916-572-2 |pages=12–5}}</ref>

The [[sleep cycle]] of alternate NREM and REM sleep takes an average of 90 minutes, occurring 4–6 times in a good night's sleep.<ref name=Peraita-Adrados2005 /><ref name=McCarley2007>{{cite journal | vauthors = McCarley RW | title = Neurobiology of REM and NREM sleep | journal = Sleep Medicine | volume = 8 | issue = 4 | pages = 302–30 | date = June 2007 | pmid = 17468046 | doi = 10.1016/j.sleep.2007.03.005 | author-link = Robert W. McCarley }}</ref> The [[American Academy of Sleep Medicine]] (AASM) divides NREM into three stages: N1, N2, and N3, the last of which is also called delta sleep or [[slow-wave sleep]].<ref>{{cite journal | vauthors = Silber MH, Ancoli-Israel S, Bonnet MH, Chokroverty S, Grigg-Damberger MM, Hirshkowitz M, Kapen S, Keenan SA, Kryger MH, Penzel T, Pressman MR, Iber C | display-authors = 6 | title = The visual scoring of sleep in adults | journal = Journal of Clinical Sleep Medicine | volume = 3 | issue = 2 | pages = 121–31 | date = March 2007 | pmid = 17557422 | doi = 10.5664/jcsm.26814 | doi-access = free }}</ref> The whole period normally proceeds in the order: N1 → N2 → N3 → N2 → REM. REM sleep occurs as a person returns to stage 2 or 1 from a deep sleep.<ref name=ninds/> There is a greater amount of deep sleep (stage N3) earlier in the night, while the proportion of REM sleep increases in the two cycles just before natural awakening.<ref name=Peraita-Adrados2005 />