Editing Sleep cycle (section)

== Length ==
[[File:Sleep Cycle.svg|thumb|100px|Schematic illustration of a normal sleep cycle]]
The standard figure given for the average length of the sleep cycle in an adult man is 90 minutes. N1 (NREM stage 1) is when the person is drowsy or awake to falling asleep. Brain waves and muscle activity start to decrease at this stage. N2 is when the person experiences a light sleep. Eye movement has stopped by this time. Brain wave frequency and muscle tonus is decreased. The heart rate and body temperature also goes down. N3 or even N4 are the most difficult stages to be awakened. Every part of the body is now relaxed, breathing, blood pressure and body temperature are reduced. The [[National Sleep Foundation]] discusses the different stages of NREM sleep and their importance. They describe REM sleep as "A unique state, in which dreams usually occur. The brain is awake and body paralyzed." This unique stage usually occurs when the person dreams.<ref name="Hartmann1968" /><ref name=McCarley2007>{{cite journal | last1 = McCarley | first1 = Robert W. | author-link = Robert W. McCarley | year = 2007| title = Neurobiology of REM and NREM sleep | journal = Sleep Medicine | volume =  8| issue = 4| pages =  302–330| doi = 10.1016/j.sleep.2007.03.005 | pmid = 17468046 }}</ref> The figure of 90 minutes for the average length of a sleep cycle was popularized by [[Nathaniel Kleitman]] around 1963.<ref name=FeinbergFloyd1979 /> Other sources give 90–110 minutes<ref name=GronfierEtAl1999 /> or 80–120 minutes.<ref name=BrandenbergerEtAl2001 />

[[File:Sleeping asian elephant.jpg|thumb|left|The captive [[Asian elephant]] (pictured) is thought to have a sleep cycle of 72 minutes.<ref name=Tobler1992>{{cite journal | last1 = Tobler | first1 = Irene | year = 1992| title = Behavioral sleep in the Asian elephant in captivity | journal = Sleep | volume =  15| issue = 1| pages =  1–12| pmid = 1557589 }}</ref>]]

In [[infant]]s, the sleep cycle lasts about 50–60 minutes; average length increases as the human grows into adulthood. In [[cat]]s, the sleep cycle lasts about 30 minutes, though it is about 12 minutes in [[rat]]s and up to 120 minutes in [[elephant]]s (In this regard, the [[ontogeny]] of the sleep cycle appears proportionate with [[metabolism|metabolic]] processes, which vary in proportion with organism size. However, shorter sleep cycles detected in some elephants complicate this theory).<ref name=McCarley2007 /><ref name=Hartmann1968 /><ref name=Tobler1992 />

The cycle can be defined as lasting from the end of one REM period to the end of the next,<ref name=Hartmann1968 /> or from the beginning of REM, or from the beginning of non-REM stage 2 (the decision of how to mark the periods makes a difference for research purposes, because of the unavoidable inclusion or exclusion of the night's first NREM or its final REM phase if directly preceding [[Wakefulness|awakening]]).<ref name=FeinbergFloyd1979>{{cite journal | last1 = Feinberg | first1 = I. | last2 = Floyd | first2 = T. C. | year = 1979| title = Systematic Trends Across the Night in Human Sleep Cycles | url = https://www.researchgate.net/publication/22916025 | journal = Psychophysiology | volume = 16 | issue = 3| pages = 283–291| doi = 10.1111/j.1469-8986.1979.tb02991.x | pmid = 220659 }}</ref>

A 7–8-hour sleep probably includes five cycles, the middle two of which tend to be longer than the first and the fourth.<ref name=FeinbergFloyd1979 /> REM takes up more of the cycle as the night goes on.<ref name=McCarley2007 /><ref>Daniel Aeschbach, "REM-sleep regulation: circadian, homeostatic, and non-REM sleep-dependent determinants"; in Mallick et al. (2011).</ref>