Editing Free-running sleep (section)

==In humans==
[[Non-24-hour sleep–wake disorder]], also referred to as ''free-running disorder'' (FRD) or ''Non-24'', is one of the [[circadian rhythm sleep disorder]]s in humans. It affects more than half<ref>{{cite book |author=Teofilo Lee-Chiong |title=Sleep: a comprehensive handbook |publisher=Wiley-Liss |location=New York |year=2006 |pages=385 |isbn=0-471-68371-X}}</ref> of people who are totally [[blindness|blind]] and a smaller number of sighted individuals.<ref name=AASM>[https://www.researchgate.net/publication/299079705_Circadian_Rhythm_Sleep_Disorders_Part_II_Advanced_Sleep_Phase_Disorder_Delayed_Sleep_Phase_Disorder_Free-Running_Disorder_and_Irregular_Sleep-Wake_Rhythm Sack RL et al. (2007) ''Circadian Rhythm Sleep Disorders: Part II, Advanced Sleep Phase Disorder, Delayed Sleep Phase Disorder, Free-Running Disorder, and Irregular Sleep–Wake Rhythm.''] {{Webarchive|url=https://web.archive.org/web/20190415142316/https://www.researchgate.net/publication/299079705_Circadian_Rhythm_Sleep_Disorders_Part_II_Advanced_Sleep_Phase_Disorder_Delayed_Sleep_Phase_Disorder_Free-Running_Disorder_and_Irregular_Sleep-Wake_Rhythm |date=2019-04-15 }} PDF,  30(11):1484-1501.</ref>

Among blind people, the cause is the inability to register, and therefore to entrain to, light cues. The many blind people who do entrain to the 24-hour light/dark cycle have eyes with functioning retinas including operative non-visual light-sensitive cells, [[ipRGCs]].<ref>{{cite journal |author=Tu, D.C. |title=Physiologic diversity and development of intrinsically photosensitive retinal ganglion cells |journal=Neuron |volume=48 |issue=6 |pages=987–99 |date=December 2005 |pmid=16364902 |doi=10.1016/j.neuron.2005.09.031 |quote=Intrinsically photosensitive retinal ganglion cells (ipRGCs) mediate numerous nonvisual phenomena, including entrainment of the circadian clock to light-dark cycles, pupillary light responsiveness, and light-regulated hormone release. |last2=Zhang |first2=D. |last3=Demas |first3=J. |display-authors=4 |last4=Slutsky |first4=Elon B. |last5=Provencio |first5=Ignacio |last6=Holy |first6=Timothy E. |last7=Van Gelder |first7=Russell N.|doi-access=free }}</ref> These [[retinal ganglion cell|ganglion cell]]s, which contain [[melanopsin]], convey their signals to the "circadian clock" via the [[retinohypothalamic tract]] (branching off from the [[optic nerve]]), linking the retina to the [[pineal gland]].<ref>{{cite journal |last=Czeisler |first=Charles A. |author2=Theresa L. Shanahan |author3=Elizabeth B. Klerman |author4=Heinz Martens |author5=Daniel J. Brotman |author6=Jonathan S. Emens |author7=Torsten Klein |author8=Joseph F. Rizzo  |date=5 January 1995 |title=Suppression of melatonin secretion in some blind patients by exposure to bright light |journal=N Engl J Med |volume=332 |issue=1 |pages=6–11 |location=USA |pmid= 7990870 |doi=10.1056/NEJM199501053320102 |quote=[T]he photic pathway used by the circadian system is functionally intact in some blind patients.|doi-access=free }}</ref><ref>{{cite web |url=http://www.endotext.org/neuroendo/neuroendo15/neuroendoframe15.htm |title=Chapter 15. The Pineal Gland and Pineal Tumours |access-date=2008-02-07 |last=Arendt |first=Josephine |date=2006-02-01 |work=Neuroendocrinology, Hypothalamus, and Pituitary |publisher=Endotext.com |pages=an E–book edited by Ashley Grossman (chapter section: Melatonin Synthesis and Metabolism) |quote=Image forming vision (rods and cones) is not required ... for synchronising/phase shifting the circadian clock. |archive-url=https://web.archive.org/web/20080209103150/http://www.endotext.org/neuroendo/neuroendo15/neuroendoframe15.htm |archive-date=2008-02-09 |url-status=dead }}</ref>

Among sighted individuals, Non-24 usually first appears in the teens or early twenties. As with [[delayed sleep phase disorder]] (DSPS or DSPD), in the absence of neurological damage due to trauma or [[stroke]], cases almost never appear after the age of 30.<ref name=AASM/> Non-24 affects more sighted males than sighted females.<ref name=AASM/> A quarter of sighted individuals with Non-24 also have an associated psychiatric condition, and a quarter of them have previously shown symptoms of DSPS.<ref name=AASM/>