Editing Free-running sleep

{{Short description|Rare, shifting sleep pattern}}
'''Free-running sleep''' is a rare [[sleep]] pattern whereby the sleep schedule of a person shifts later every day.<ref>{{cite web |title=Circadian Rhythm Sleep Disorders |url=https://aasm.org/resources/factsheets/crsd.pdf |website=aasm |access-date=4 July 2024 |archive-date=11 June 2016 |archive-url=https://web.archive.org/web/20160611134934/http://www.aasmnet.org/resources/factsheets/crsd.pdf |url-status=live }}</ref> It occurs as the [[sleep disorder]] [[non-24-hour sleep–wake disorder]] or artificially as part of experiments used in the study of [[circadian rhythm]]s and other rhythms in [[biology]]. Study subjects are shielded from all time cues, often by a constant light [[protocol (natural sciences)|protocol]], by a constant dark protocol or by the use of light/dark conditions to which the organism cannot entrain such as the ultrashort protocol of one hour dark and two hours light. Also, limited amounts of food may be made available at short intervals so as to avoid entrainment to mealtimes. Subjects are thus forced to live by their internal circadian "clocks".

==Background==
The individual's or animal's circadian phase can be known only by the monitoring of some kind of output of the circadian system, the internal "body clock". The researcher can precisely determine, for example, the daily cycles of [[gene]] activity, [[body temperature]], [[blood pressure]], [[hormone]] secretion or [[sleep]] and activity/[[alertness]]. Alertness in humans can be determined by many kinds of verbal and non-verbal tests, whereas alertness in animals can usually be assessed by observing physical activity (for example, of wheel-running in [[rodent]]s).

When animals or people ''free-run'', experiments can be done to see what sort of signals, known as [[zeitgeber]]s, are effective in entrainment. Also, much work has been done to see how long or short a circadian cycle can be entrained to various organisms. For example, some animals can be entrained to a 22-hour day, but they can not be entrained to a 20-hour day. In recent studies funded by the U.S. space industry, it has been shown that most humans can be entrained to a 23.5-hour day and to a 24.65-hour day.<ref>{{cite journal |last=Scheer |first=Frank A.J.L. |author2=Kenneth P. Wright Jr. |author3=Richard E. Kronauer |author4=Charles A. Czeisler |date=2007-08-08 |title=Plasticity of the Intrinsic Period of the Human Circadian Timing System |journal=PLOS ONE |volume=2 |pages=e721 |quote=[E]xposure to moderately bright light (~450 lux; ~1.2 W/m<sup>2</sup>) for the second or first half of the scheduled wake episode is effective for entraining individuals to the 24.65-h Martian sol and a 23.5-h day length, respectively. |doi=10.1371/journal.pone.0000721 |pmid=17684566 |issue=1 |pmc=1934931 |bibcode=2007PLoSO...2..721S |editor1-last=Nicolelis |editor1-first=Miguel|doi-access=free }}</ref>

The effect of unintended time cues is called ''masking'' and can totally confound experimental results. Examples of masking are morning rush traffic audible to the subjects, or researchers or maintenance staff visiting subjects on a regular schedule.

==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/>

==See also==
* [[Circadian rhythm]]
* [[Circadian rhythm sleep disorder]]

==References==
{{Reflist}}

==External links==
* [http://www.supermemo.com/articles/sleep.htm#Free%20running%20sleep A collection of articles about sleep] by Piotr A. Wozniak, July 2000
{{Light Ethology}}

[[Category:Sleep]]
[[Category:Circadian rhythm]]