Editing Sleep cycle

{{short description|Oscillation between the slow-wave and REM phases of sleep}}
{{For|the album|Sleep Cycle}}
[[File:Hypro zyklus 1 en 103.svg|thumb|345px|Sample [[hypnogram]] showing one sleep cycle (the first of the night) from NREM through REM]]
The '''sleep cycle''' is an oscillation between the [[slow-wave sleep|slow-wave]] and [[rapid eye movement sleep|REM]] (paradoxical) phases of [[sleep]]. It is sometimes called the '''[[ultradian]] sleep cycle''', '''sleep–dream cycle''', or '''REM-NREM cycle''', to distinguish it from the [[circadian rhythm|circadian]] alternation between sleep and [[wakefulness]]. In humans, this cycle takes 70 to 110 minutes (90 ± 20 minutes).<ref>{{Cite web|title=What Happens During Sleep? {{!}} UPMC Sleep Medicine Resources|url=https://www.upmc.com/services/pulmonology/our-services/sleep-medicine/resources/during-sleep|access-date=2021-09-24|website=UPMC {{!}} Life Changing Medicine|language=en}}</ref>  Within the sleep of adults and infants there are cyclic fluctuations between quiet and active sleep. These fluctuations may persist during wakefulness as rest-activity cycles but are less easily discerned. <ref>{{Cite journal |url=https://publications.aap.org/pediatrics/article/43/1/65/44911/SLEEP-CYCLE-CHARACTERISTICS-IN-INFANTS?autologincheck=redirected |access-date=2023-11-18 |journal=Pediatrics |doi=10.1542/peds.43.1.65 |title=Sleep Cycle Characteristics in Infants |date=1969 |last1=Stern |first1=Evelyn |last2=Parmelee |first2=Arthur H. |last3=Akiyama |first3=Yoshio |last4=Schultz |first4=Marvin A. |last5=Wenner |first5=Waldemar H. |volume=43 |pages=65–70 |url-access=subscription }}</ref>

== Characteristics ==
[[Electroencephalography]] shows the timing of sleep cycles by virtue of the marked distinction in [[brainwave]]s manifested during REM and non-REM sleep. [[Delta wave]] activity, correlating with slow-wave (deep) sleep, in particular shows regular oscillations throughout a good night's sleep. Secretions of various [[hormone]]s, including [[renin]], [[growth hormone]], and [[prolactin]], correlate positively with delta-wave activity, while secretion of [[thyroid-stimulating hormone]] correlates inversely.<ref name=GronfierEtAl1999>{{cite journal | last1 = Gronfier | first1 = Claude | last2 = Simon | first2 = Chantal | last3 = Piquard | first3 = François | last4 = Ehrhart | first4 = Jean | last5 = Brandenberger | first5 = Gabrielle | year = 1999|title=Neuroendocrine Processes Underlying Ultradian Sleep Regulation in Man | journal = Journal of Clinical Endocrinology & Metabolism | volume =  84| issue = 8| pages =  2686–2690| doi = 10.1210/jcem.84.8.5893 | pmid = 10443660 | doi-access = free }}</ref> [[Heart rate variability]], well known to increase during REM, predictably also correlates inversely with delta-wave oscillations over the ~90-minute cycle.<ref name=BrandenbergerEtAl2001>{{cite journal | last1 = Brandenberger | first1 = Gabrielle | last2 = Erhart | first2 = Jean | last3 = Piquard | first3 = François | last4 = Simon | first4 = Chantal | year = 2001| title = Inverse coupling between ultradian oscillations in delta wave activity and heart rate variability during sleep | url = https://pdfs.semanticscholar.org/db12/f3a46522450060675caad0a86d496434f01d.pdf | archive-url = https://web.archive.org/web/20170804113139/https://pdfs.semanticscholar.org/db12/f3a46522450060675caad0a86d496434f01d.pdf | url-status = dead | archive-date = 2017-08-04 | journal = Clinical Neurophysiology | volume =  112| issue = 6| pages =  992–996| doi = 10.1016/S1388-2457(01)00507-7 | pmid = 11377256 | s2cid = 206133162 }}</ref>

In order to determine in which stage of sleep the asleep subject is, electroencephalography is combined with other devices used for this differentiation. EMG ([[electromyography]]) is a crucial method to distinguish between sleep phases: for example, a decrease of [[muscle tone]] is in general a characteristic of the transition from wake to sleep,<ref name="Chase, M. H. 1990">{{cite journal|last1=Chase|first1=M. H.|last2=Morales|first2=F. R.|year=1990|title=The atonia and myoclonia of active (REM) sleep|journal=[[Annual Review of Psychology]]|volume=41|issue=1|pages=557–584|doi=10.1146/annurev.ps.41.020190.003013|pmid=1968326}}</ref><ref>Kleitman, N. (1963). Sleep and Wakefulness Chicago, Univ. ''Chicago Jfress''</ref> and during REM sleep, there is a state of muscle atonia (paralysis), resulting in an absence of signals in the EMG.<ref name="Chase, M. H. 1990"/>

[[Electrooculography|EOG (electrooculography)]], the measure of the eyes’ movement, is the third method used in the sleep architecture measurement;<ref>Berry, R. B., & Wagner, M. H. (2014). ''Sleep Medicine Pearls E-Book''. Elsevier Health Sciences.</ref> for example, REM sleep, as the name indicates, is characterized by a rapid eye movement pattern, visible thanks to the EOG.<ref>ber C., Ancoli-Israel S., Chesson A., and Quan SF. in The AASM Manual for the Scoring of Sleep and Associated Events:  Rules, Terminology and Technical Specifications, 1st. Ed.:  Westchester, Illinois:  American Academy of Sleep Medicine; 2007.</ref>

Moreover, methods based on cardiorespiratory parameters are also effective in the analysis of sleep architecture—if they are associated with the other aforementioned measurements (such as electroencephalography, electrooculography and the electromyography).<ref>Tataraidze, A., Korostovtseva, L., Anishchenko, L., Bochkarev, M., & Sviryaev, Y. (2016,). Sleep architecture measurement based on cardiorespiratory parameters.</ref>

[[Homeostasis|Homeostatic]] functions, especially [[thermoregulation]], occur normally during non-REM sleep, but not during REM sleep. Thus, during REM sleep, body temperature tends to drift away from its mean level, and during non-REM sleep, to return to normal. Alternation between the stages therefore maintains body temperature within an acceptable range.<ref>Pier Luigi Parmeggiani, "Modulation of body core temperature in NREM sleep and REM sleep"; in Mallick et al. (2011).</ref>

In humans, the transition between non-REM and REM is abrupt; in other animals, it is less so.<ref name=McCarley2007 />

Researchers have proposed different models to elucidate the undoubtedly complex rhythm of electrochemical processes that result in the regular alternation of REM and NREM sleep. [[Monoamine]]s are active during NREMS, but not REMS, whereas [[acetylcholine]] is more active during REMS. The [[activation-synthesis hypothesis|reciprocal interaction]] model proposed in the 1970s suggested a cyclic give-and-take between these two systems. More recent theories such as the "flip-flop" model, proposed in the 2000s, include the regulatory role of an inhibitory neurotransmitter [[gamma-aminobutyric acid]] (GABA).<ref>James T. McKenna, Lichao Chen, & Robert McCarley, "Neuronal models of REM-sleep control: evolving concepts"; in Mallick et al. (2011).</ref>

== 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>

== Awakening ==
Unprovoked awakening occurs most commonly during or after a period of REM sleep, as [[body temperature]] is rising.<ref>{{cite journal | last1 = Åkerstedt | first1 = Torbjorn | last2 = Billiard | first2 = Michel | last3 = Bonnet | first3 = Michael | last4 = Ficca | first4 = Gianluca | last5 = Garma | first5 = Lucile | last6 = Mariotti | first6 = Maurizio | last7 = Salzarulo | first7 = Piero | last8 = Schulz | first8 = Hartmut | year = 2002|url=https://www.researchgate.net/publication/10948058 |title=Awakening from sleep | journal = Sleep Medicine Reviews | volume =  6| issue = 4| pages =  267–286| doi = 10.1053/smrv.2001.0202 | pmid = 12531132 | doi-access = free }}</ref>

== 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 />

== Alteration ==

The sleep cycle has proven resistant to systematic alteration by [[drug]]s. Although some drugs shorten REM periods, they do not abolish the underlying rhythm. Deliberate REM deprivation shortens the cycle temporarily, as the brain moves into REM sleep more readily (the "[[REM rebound]]") in an apparent correction for the deprivation.<ref name=Hartmann1968 /> There are various methods to control the alterations of sleep cycles:
* Switching off all artificial lights:<ref>{{cite journal |last1=Blume |first1=Christine |last2=Garbazza |first2=Corrado |last3=Spitschan |first3=Manuel |title=Effects of light on human circadian rhythms, sleep and mood |journal=Somnologie |pages=147–156 |doi=10.1007/s11818-019-00215-x |date=2019|volume=23 |issue=3 |pmid=31534436 |pmc=6751071}}</ref> Since the natural production of melatonin can be suppressed by bright light, exposure to light–even after sunset<ref>{{cite journal |last1=L |first1=Tähkämö |last2=T |first2=Partonen |last3=Ak |first3=Pesonen |title=Systematic Review of Light Exposure Impact on Human Circadian Rhythm |journal=Chronobiology International |language=en |date=2019|volume=36 |issue=2 |pages=151–170 |doi=10.1080/07420528.2018.1527773 |pmid=30311830 |s2cid=52960410 |doi-access=free|hdl=10138/311143 |hdl-access=free }}</ref>–may prevent the body from feeling sleepy (and hence entering the sleep phase).
* Meditation and relaxation techniques<ref>{{cite journal |last1=P |first1=Spagnoli |last2=C |first2=Balducci |last3=M |first3=Fabbri |last4=D |first4=Molinaro |last5=G |first5=Barbato |title=Workaholism, Intensive Smartphone Use, and the Sleep-Wake Cycle: A Multiple Mediation Analysis |journal=International Journal of Environmental Research and Public Health |language=en |date=20 September 2019|volume=16 |issue=19 |page=3517 |doi=10.3390/ijerph16193517 |pmid=31547191 |pmc=6801767|doi-access=free }}</ref> 
* Staying away from caffeine before bedtime:<ref>{{cite journal |last1=Anderson |first1=Jason R. |last2=Hagerdorn |first2=Payton L. |last3=Gunstad |first3=John |last4=Spitznagel |first4=Mary Beth |title=Using Coffee to Compensate for Poor Sleep: Impact on Vigilance and Implications for Workplace Performance |url=https://pubmed.ncbi.nlm.nih.gov/29866304/?from_term=coffee+sleep&from_pos=3 |journal=Applied Ergonomics |language=en |date=2018|volume=70 |pages=142–147 |doi=10.1016/j.apergo.2018.02.026 |pmid=29866304 }}</ref> This ensures that the body is not under the stimulant effects of caffeine while trying to sleep.

== See also ==

* [[Biphasic and polyphasic sleep]]
* [[Circadian rhythm]]

== References ==
{{Reflist|refs=

<ref name=Hartmann1968>{{cite journal | last = Hartmann | first = Ernest | author-link = Ernest Hartmann | title = The 90-Minute Sleep-Dream Cycle | journal = Archives of General Psychiatry | volume = 18 | issue = 3 | pages = 280–6| date = March 1968 | doi = 10.1001/archpsyc.1968.01740030024004 | pmid = 5638533 }}</ref>

}}

=== Bibliography ===
* Mallick, B. N.; S. R. Pandi-Perumal; Robert W. McCarley; and Adrian R. Morrison (2011). ''Rapid Eye Movement Sleep: Regulation and Function''. Cambridge University Press. {{ISBN|978-0-521-11680-0}}
*Nir, and Tononi, "Dreaming and the Brain: from Phenomenology to Neurophsiology." ''Trends in Cognitive Sciences'', vol. 14, no. 2, 2010, pp.&nbsp;88–100. 
*Varela, F., Engel, J., Wallace, B., & Thupten Jinpa. (1997). ''Sleeping, dreaming, and dying: An exploration of consciousness with the Dalai Lama''.

[[Category:Sleep physiology]]
[[Category:Chronobiology]]