Editing Sleep (section)

== Physiology ==
{{Main|Neuroscience of sleep}}
The most pronounced physiological changes in sleep occur in the brain.<ref name=MaquetEtAl2006>{{cite book |doi=10.1142/9781860947186_0006 |chapter=Brain Imaging on Passing to Sleep |title=The Physiologic Nature of Sleep |chapter-url=https://books.google.com/books?id=cxFqDQAAQBAJ&pg=PA123 |year=2005 | vauthors = Maquet PA, Sterpenich V, Albouy G, Dang-Vu T, Desseilles M, Boly M, Ruby P, Laureys S, Peigneux P | display-authors = 6 |pages=123–137 |publisher=Published by Imperial College Press and Distributed by World Scientific Publishing Co. |isbn=978-1-86094-557-1 }}</ref> The brain uses significantly less energy during sleep than it does when awake, especially during non-REM sleep. In areas with reduced activity, the brain restores its supply of [[adenosine triphosphate]] (ATP), the molecule used for short-term storage and transport of energy.<ref>[[#Brown|Brown]], pp. 1118–1119: "Compared with wakefulness, sleep reduces brain energy demands, as suggested by the 44% reduction in the cerebral metabolic rate (CMR) of glucose (791) and a 25% reduction in the CMR of O<sub>2</sub> (774) during sleep."</ref> In quiet waking, the brain is responsible for 20% of the body's energy use, thus this reduction has a noticeable effect on overall energy consumption.<ref name=Siegel2008>{{cite journal | vauthors = Siegel JM | title = Do all animals sleep? | journal = Trends in Neurosciences | volume = 31 | issue = 4 | pages = 208–13 | date = April 2008 | pmid = 18328577 | doi = 10.1016/j.tins.2008.02.001 | pmc = 8765194 | s2cid = 6614359 }}</ref>

Sleep increases the [[sensory threshold]]. In other words, sleeping persons perceive fewer stimuli, but can generally still respond to loud noises and other salient sensory events.<ref name="Siegel2008"/><ref name=MaquetEtAl2006 />

During [[slow-wave sleep]], humans secrete bursts of [[growth hormone]]. All sleep, even during the day, is associated with the secretion of [[prolactin]].<ref name=VanCauterSpiegel1999>{{cite book | vauthors = Van Cauter E, Spiegel K | date = 1999 | chapter = Circadian and Sleep Control of Hormonal Secretions | veditors = Zee PC, Turek FW | title = Regulation of Sleep and Circadian Rhythms | pages = 397–425 }}</ref>

Key physiological methods for monitoring and measuring changes during sleep include [[electroencephalography]] (EEG) of [[neural oscillation|brain waves]], [[electrooculography]] (EOG) of eye movements, and [[electromyography]] (EMG) of [[skeletal muscle]] activity. Simultaneous collection of these measurements is called [[polysomnography]], and can be performed in a specialized [[sleep medicine#Diagnostic methods|sleep laboratory]].<ref>[[#Brown|Brown]], p. 1087.</ref><ref name=Peraita-Adrados2005>{{cite book |doi=10.1142/9781860947186_0005 |chapter=Electroencephalography, Polysomnography, and Other Sleep Recording Systems |chapter-url=https://books.google.com/books?id=cxFqDQAAQBAJ&pg=PA103 |title=The Physiologic Nature of Sleep |year=2005 | vauthors = Peraita-Adrados R |pages=103–122 |publisher=Published by Imperial College Press and Distributed by World Scientific Publishing Co. |isbn=978-1-86094-557-1 }}</ref> Sleep researchers also use simplified [[electrocardiography]] (EKG) for cardiac activity and [[actigraphy]] for motor movements.<ref name=Peraita-Adrados2005 />

=== Brain waves in sleep ===
The electrical activity seen on an EEG represents brain waves. The amplitude of EEG waves at a particular frequency corresponds to various points in the sleep-wake cycle, such as being asleep, being awake, or falling asleep.<ref>{{citation|vauthors=Borbély AA, Daan S, Wirz-Justice A, Deboer T|title= The two-process model of sleep regulation: a reappraisal|journal= J Sleep Res | volume=25|issue=2|pages=131–43|doi= 10.1111/jsr.12371 |date=14 January 2016 |pmid=26762182|s2cid= 206156163|url= https://www.zora.uzh.ch/id/eprint/121344/1/TPM%20a%20reappraisal%2025-11-15%20Figs%20inserted%20in%20text-2.pdf}}</ref> Alpha, beta, theta, gamma, and delta waves are all seen in the different stages of sleep. Each waveform maintains a different frequency and amplitude. Alpha waves are seen when a person is in a resting state, but is still fully conscious. Their eyes may be closed and all of their body is resting and relatively still, where the body is starting to slow down. Beta waves take over alpha waves when a person is at attention, as they might be completing a task or concentrating on something. Beta waves consist of the highest of frequencies and the lowest of amplitude, and occur when a person is fully alert. Gamma waves are seen when a person is highly focused on a task or using all their concentration. Theta waves occur during the period of a person being awake, and they continue to transition into Stage 1 of sleep and in stage 2. Delta waves are seen in stages 3 and 4 of sleep when a person is in their deepest of sleep.<ref>{{citation|vauthors=Posada-Quintero HF, Reljin N, Bolkhovsky JB, Orjuela-Cañón AD, Chon KH|title=Brain Activity Correlates With Cognitive Performance Deterioration During Sleep Deprivation. |journal=Front. Neurosci. |date= 19 September 2019 |volume=13 |page=1001 |doi=10.3389/fnins.2019.01001|pmid=31607847 |pmc=6761229 |doi-access=free }}</ref>

===Non-REM and REM sleep===
Sleep is divided into two broad types: [[non-rapid eye movement sleep|non-rapid eye movement]] (non-REM or NREM) sleep and [[rapid eye movement sleep|rapid eye movement]] (REM) sleep. Non-REM and REM sleep are so different that physiologists identify them as distinct behavioral states. Non-REM sleep occurs first and after a transitional period is called [[slow-wave sleep]] or deep sleep. During this phase, body temperature and heart rate fall, and the brain uses less energy.<ref name=MaquetEtAl2006 /> REM sleep, also known as paradoxical sleep, represents a smaller portion of total sleep time. It is the main occasion for dreams (or [[nightmare]]s), and is associated with desynchronized and fast brain waves, eye movements, loss of muscle tone,<ref name="ninds">{{cite web |title=Brain Basics: Understanding Sleep |url=https://www.ninds.nih.gov/Disorders/Patient-Caregiver-Education/Understanding-sleep |website=National Institute of Neurological Disorders and Stroke}}</ref> and suspension of [[homeostasis]].<ref>{{cite book | veditors = Parmeggiani PL | date = 2011 | title = Systemic Homeostasis and Poikilostasis in Sleep: Is REM Sleep a Physiological Paradox? | location = London | publisher = Imperial College Press | isbn = 978-1-94916-572-2 |pages=12–5}}</ref>

The [[sleep cycle]] of alternate NREM and REM sleep takes an average of 90 minutes, occurring 4–6 times in a good night's sleep.<ref name=Peraita-Adrados2005 /><ref name=McCarley2007>{{cite journal | vauthors = McCarley RW | title = Neurobiology of REM and NREM sleep | journal = Sleep Medicine | volume = 8 | issue = 4 | pages = 302–30 | date = June 2007 | pmid = 17468046 | doi = 10.1016/j.sleep.2007.03.005 | author-link = Robert W. McCarley }}</ref> The [[American Academy of Sleep Medicine]] (AASM) divides NREM into three stages: N1, N2, and N3, the last of which is also called delta sleep or [[slow-wave sleep]].<ref>{{cite journal | vauthors = Silber MH, Ancoli-Israel S, Bonnet MH, Chokroverty S, Grigg-Damberger MM, Hirshkowitz M, Kapen S, Keenan SA, Kryger MH, Penzel T, Pressman MR, Iber C | display-authors = 6 | title = The visual scoring of sleep in adults | journal = Journal of Clinical Sleep Medicine | volume = 3 | issue = 2 | pages = 121–31 | date = March 2007 | pmid = 17557422 | doi = 10.5664/jcsm.26814 | doi-access = free }}</ref> The whole period normally proceeds in the order: N1 → N2 → N3 → N2 → REM. REM sleep occurs as a person returns to stage 2 or 1 from a deep sleep.<ref name=ninds/> There is a greater amount of deep sleep (stage N3) earlier in the night, while the proportion of REM sleep increases in the two cycles just before natural awakening.<ref name=Peraita-Adrados2005 />

=== Awakening ===
{{Redirect|Waking up }}
{{Further|Wakefulness|Ascending reticular activating system}}
[[File:1900 The Awakening.jpg|thumb|"The Awakening", an illustration to writing by [[Leo&nbsp;Tolstoy]]
]]
Awakening can mean the end of sleep, or simply a moment to survey the environment and readjust body position before falling back asleep. Sleepers typically awaken soon after the end of a REM phase or sometimes in the middle of REM. Internal [[Circadian clock|circadian indicators]], along with a successful reduction of homeostatic sleep need, typically bring about awakening and the end of the sleep cycle.<ref name=AkerstedtEtAl2002>{{cite journal | vauthors = Akerstedt T, Billiard M, Bonnet M, Ficca G, Garma L, Mariotti M, Salzarulo P, Schulz H | display-authors = 6 | title = Awakening from sleep | journal = Sleep Medicine Reviews | volume = 6 | issue = 4 | pages = 267–86 | date = August 2002 | pmid = 12531132 | doi = 10.1053/smrv.2001.0202 | doi-access = free }}</ref> Awakening involves heightened electrical activation in the brain, beginning with the [[thalamus]] and spreading throughout the [[Cerebral cortex|cortex]].<ref name=AkerstedtEtAl2002 />

On a typical night of sleep, there is not much time that is spent in the waking state. In various sleep studies that have been conducted using the electroencephalography, it has been found that females are awake for 0–1% during their nightly sleep while males are awake for 0–2% during that time. In adults, wakefulness increases, especially in later cycles. One study found 3% awake time in the first ninety-minute sleep cycle, 8% in the second, 10% in the third, 12% in the fourth, and 13–14% in the fifth. Most of this awake time occurred shortly after REM sleep.<ref name=AkerstedtEtAl2002 />

Today, many humans wake up with an [[alarm clock]];<ref name=RonnenbergEtAl2007>{{cite journal | vauthors = Roenneberg T, Kuehnle T, Juda M, Kantermann T, Allebrandt K, Gordijn M, Merrow M | title = Epidemiology of the human circadian clock | journal = Sleep Medicine Reviews | volume = 11 | issue = 6 | pages = 429–38 | date = December 2007 | pmid = 17936039 | doi = 10.1016/j.smrv.2007.07.005 | url = https://pure.rug.nl/ws/files/6712602/2007SleepMedRevRoenneberg.pdf | hdl = 11370/65d6f03a-88cd-405c-a067-4afbc1b9ba9d | s2cid = 11628329 | hdl-access = free }}</ref> however, people can also reliably wake themselves up at a specific time with no need for an alarm.<ref name=AkerstedtEtAl2002 /> Many sleep quite differently on workdays versus days off, a pattern which can lead to chronic circadian desynchronization.<ref name=WaterhouseEtAl2012>{{cite journal | vauthors = Waterhouse J, Fukuda Y, Morita T | title = Daily rhythms of the sleep-wake cycle | journal = Journal of Physiological Anthropology | volume = 31 | issue = 5 | pages = 5 | date = March 2012 | pmid = 22738268 | pmc = 3375033 | doi = 10.1186/1880-6805-31-5 | doi-access = free }}</ref><ref name=RonnenbergEtAl2007 /> Many people regularly look at television and other screens before going to bed, a factor which may exacerbate disruption of the circadian cycle.<ref name="ChangAM" /><ref>{{cite journal | vauthors = Basner M, Dinges DF | title = Dubious bargain: trading sleep for Leno and Letterman | journal = Sleep | volume = 32 | issue = 6 | pages = 747–52 | date = June 2009 | pmid = 19544750 | pmc = 2690561 | doi = 10.1093/sleep/32.6.747 }}</ref> Scientific studies on sleep have shown that sleep stage at awakening is an important factor in amplifying [[sleep inertia]].<ref>{{cite journal | vauthors = Tassi P, Muzet A | title = Sleep inertia | journal = Sleep Medicine Reviews | volume = 4 | issue = 4 | pages = 341–353 | date = August 2000 | pmid = 12531174 | doi = 10.1053/smrv.2000.0098 }}</ref>

Determinants of [[alertness]] after waking up include quantity/quality of the sleep, physical activity the day prior, a carbohydrate-rich breakfast, and a low [[Blood sugar level|blood glucose]] response to it.<ref>{{cite journal |last1=Vallat |first1=Raphael |last2=Berry |first2=Sarah E. |last3=Tsereteli |first3=Neli |last4=Capdevila |first4=Joan |last5=Khatib |first5=Haya Al |last6=Valdes |first6=Ana M. |last7=Delahanty |first7=Linda M. |last8=Drew |first8=David A. |last9=Chan |first9=Andrew T. |last10=Wolf |first10=Jonathan |last11=Franks |first11=Paul W. |last12=Spector |first12=Tim D. |last13=Walker |first13=Matthew P. |title=How people wake up is associated with previous night's sleep together with physical activity and food intake |journal=Nature Communications |date=19 November 2022 |volume=13 |issue=1 |pages=7116 |doi=10.1038/s41467-022-34503-2 |pmid=36402781 |pmc=9675783 |bibcode=2022NatCo..13.7116V |language=en |issn=2041-1723|doi-access=free}}</ref>