Editing Sleep cycle (section)

== Continuation during wakefulness ==
[[Ernest Hartmann]] discovered in 1968 that humans seem to continue a roughly 90-minute ultradian rhythm throughout a 24-hour day, whether they are asleep or awake.<ref name=Hartmann1968 /> According to this hypothesis, during the period of this cycle corresponding with REM, people tend to [[daydream]] more and show less [[muscle tone]].<ref>Ekkehard Othmer, Mary P. Hayden, and Robert Segelbaum, "Encephalic Cycles during Sleep and Wakefulness in Humans: a 24-Hour Pattern" ([https://www.jstor.org/stable/1726608 JSTOR]); ''Science'' 164(3878), 25 April 1969.</ref> Kleitman and others following have referred to this rhythm as the [[basic rest–activity cycle]], of which the "sleep cycle" would be a manifestation.<ref name=FeinbergFloyd1979 /><ref>{{cite journal | last1 = Kleitman | first1 = Nathaniel | year = 1982| title = Basic Rest-Activity Cycle—22 Years Later | url = https://academic.oup.com/sleep/article/5/4/311/2753285/Basic-Rest-Activity-Cycle-22-Years-Later | journal = Sleep | volume =  5| issue = 4| pages =  311–317| doi = 10.1093/sleep/5.4.311 | pmid = 6819628 | doi-access = free | url-access = subscription }}</ref> A difficulty for this theory is the fact that a long non-REM phase almost always precedes REM, regardless of when in the cycle a person falls asleep.<ref name=FeinbergFloyd1979 />