Editing Sleep (section)

== Functions ==
{{Unsolved|biology|What is the biological function of sleep?}}

===Restoration===
{{Further|Glymphatic system}}

The sleeping brain has been shown to remove metabolic end products at a faster rate than during an awake state, by increasing the flow of cerebrospinal fluid during sleep.<ref name="Xie2013">{{cite journal |display-authors=6 |vauthors=Xie L, Kang H, Xu Q, Chen MJ, Liao Y, Thiyagarajan M, O'Donnell J, Christensen DJ, Nicholson C, Iliff JJ, Takano T, Deane R, Nedergaard M |date=17 October 2013 |title=Sleep drives metabolite clearance from the adult brain |journal=Science |volume=342 |issue=6156 |pages=373–7 |bibcode=2013Sci...342..373X |doi=10.1126/science.1241224 |issn=0036-8075 |pmc=3880190 |pmid=24136970}}</ref> The mechanism for this removal appears to be the [[glymphatic system]], a system that does for the brain what the lymphatic system does for the body.<ref name="Xie2013" /><ref name="ntyimes"/> Further research has shown that the glymphatic system is driven by pulses of hormones that in turn create surges in blood flow that cause the cerebrospinal fluid to flow, carrying away metabolites.<ref name="science1">{{cite news|last=Leslie|first=Mitch|title=Scientists uncover how the brain washes itself during sleep|url=https://www.science.org/content/article/scientists-uncover-how-brain-washes-itself-during-sleep?utm_source=Live+Audience&utm_campaign=b50db22312-nature-briefing-daily-20250108_COPY_01&utm_medium=email&utm_term=0_b27a691814-b50db22312-51308676|newspaper=[[Science (journal)|Science]]|accessdate=10 January 2025|date=8 January 2025|quote=She called it the glymphatic system, a nod to its dependence on glial cells}}</ref>

Sleep may facilitate the synthesis of molecules that help repair and protect the brain from metabolic end products generated during waking.<ref>{{cite journal |vauthors=Siegel JM |date=October 2005 |title=Clues to the functions of mammalian sleep |journal=Nature |volume=437 |issue=7063 |pages=1264–71 |bibcode=2005Natur.437.1264S |doi=10.1038/nature04285 |pmc=8760626 |pmid=16251951 |s2cid=234089}}</ref> [[Anabolic]] hormones, such as [[growth hormone]]s, are secreted preferentially during sleep. The brain concentration of [[glycogen]] increases during sleep, and is depleted through metabolism during wakefulness.<ref name="CespuglioEtAl2005" />

The human organism physically restores itself during sleep, occurring mostly during [[slow-wave sleep]] during which body temperature, heart rate, and brain oxygen consumption decrease. In both the brain and body, the reduced rate of [[metabolism]] enables countervailing restorative processes.<ref name=CespuglioEtAl2005>Raymond Cespuglio, Damien Colas, & Sabine Gautier-Sauvigné, "Energy Processes Underlying the Sleep Wake Cycle"; Chapter 1 in Parmeggiani & Velluti (2005).</ref> While the body benefits from sleep, the brain actually requires sleep for restoration, whereas these processes can take place during quiescent waking in the rest of the body.<ref>{{cite journal | vauthors = Eugene AR, Masiak J | title = The Neuroprotective Aspects of Sleep | journal = MEDtube Science | volume = 3 | issue = 1 | pages = 35–40 | date = March 2015 | pmid = 26594659 | pmc = 4651462 }}</ref> The essential function of sleep may be its restorative effect on the brain: "Sleep is of the brain, by the brain and for the brain."<ref name=Hobson2005>{{cite journal | vauthors = Hobson JA | title = Sleep is of the brain, by the brain and for the brain | journal = Nature | volume = 437 | issue = 7063 | pages = 1254–1256 | date = October 2005 | pmid = 16251949 | doi = 10.1038/nature04283 | s2cid = 1055112 | bibcode = 2005Natur.437.1254H }}</ref> Furthermore, this includes almost any brain, no matter how small: sleep is observed to be a necessary behavior across most of the animal kingdom, including some of the least cognitively advanced animals, implying that sleep is essential to the most fundamental brain processes, i.e. neuronal firing.  This shows that sleep is vital even when there is no need for other functions of sleep, such as [[memory consolidation]] or dreaming.<ref name=Joiner2016/>

=== Memory processing ===
{{Further|Sleep and memory|Neuroscience of sleep|Sleep and learning|Page 4=Neuroscience of sleep}}

It has been widely accepted that sleep must support the formation of long-term memory, and generally increasing previous learning and experiences recalls. However, its benefit seems to depend on the phase of sleep and the type of memory.<ref>{{cite journal | vauthors = Plihal W, Born J | title = Effects of early and late nocturnal sleep on declarative and procedural memory | journal = Journal of Cognitive Neuroscience | volume = 9 | issue = 4 | pages = 534–47 | date = July 1997 | pmid = 23968216 | doi = 10.1162/jocn.1997.9.4.534 | s2cid = 3300300 }}</ref> For example, declarative and procedural memory-recall tasks applied over early and late nocturnal sleep, as well as wakefulness controlled conditions, have been shown that declarative memory improves more during early sleep (dominated by SWS) while procedural memory during late sleep (dominated by REM sleep) does so.<ref name=":02">{{cite journal | vauthors = Rasch B, Büchel C, Gais S, Born J | title = Odor cues during slow-wave sleep prompt declarative memory consolidation | journal = Science | volume = 315 | issue = 5817 | pages = 1426–9 | date = March 2007 | pmid = 17347444 | doi = 10.1126/science.1138581 | s2cid = 19788434 | bibcode = 2007Sci...315.1426R }}</ref><ref name=":13">{{cite journal | vauthors = Born J, Wilhelm I | title = System consolidation of memory during sleep | journal = Psychological Research | volume = 76 | issue = 2 | pages = 192–203 | date = March 2012 | pmid = 21541757 | pmc = 3278619 | doi = 10.1007/s00426-011-0335-6 }}</ref>

With regard to declarative memory, the functional role of SWS has been associated with hippocampal replays of previously encoded neural patterns that seem to facilitate long-term memory consolidation.<ref name=":02" /><ref name=":13" /> This assumption is based on the active system consolidation hypothesis, which states that repeated reactivations of newly encoded information in the hippocampus during slow oscillations in NREM sleep mediate the stabilization and gradual integration of declarative memory with pre-existing knowledge networks on the cortical level.<ref>{{cite journal | vauthors = Diekelmann S, Born J | title = The memory function of sleep | journal = Nature Reviews. Neuroscience | volume = 11 | issue = 2 | pages = 114–26 | date = February 2010 | pmid = 20046194 | doi = 10.1038/nrn2762 | s2cid = 1851910 }}</ref> It assumes the hippocampus might hold information only temporarily and in a fast-learning rate, whereas the neocortex is related to long-term storage and a slow-learning rate.<ref name=":02" /><ref name=":13" /><ref name=":1">{{cite journal | vauthors = Rasch B, Born J | title = About sleep's role in memory | journal = Physiological Reviews | volume = 93 | issue = 2 | pages = 681–766 | date = April 2013 | pmid = 23589831 | pmc = 3768102 | doi = 10.1152/physrev.00032.2012 }}</ref><ref name=":22">{{cite journal | vauthors = Schreiner T, Rasch B | title = Boosting Vocabulary Learning by Verbal Cueing During Sleep | journal = Cerebral Cortex | volume = 25 | issue = 11 | pages = 4169–79 | date = November 2015 | pmid = 24962994 | doi = 10.1093/cercor/bhu139 | doi-access = free }}</ref><ref name=":52">{{cite journal | vauthors = Schreiner T, Rasch B | title = The beneficial role of memory reactivation for language learning during sleep: A review | journal = Brain and Language | volume = 167 | pages = 94–105 | date = April 2017 | pmid = 27036946 | doi = 10.1016/j.bandl.2016.02.005 | url = https://doc.rero.ch/record/323249/files/schreinerrasch2017_brainlang.pdf | s2cid = 3377186 }}</ref> This dialogue between the hippocampus and neocortex occurs in parallel with hippocampal [[Sharp waves and ripples|sharp-wave ripples]] and [[Sleep spindle|thalamo-cortical spindles]], synchrony that drives the formation of the spindle-ripple event which seems to be a prerequisite for the formation of long-term memories.<ref name=":13" /><ref name=":1" /><ref name=":52" /><ref name=":32">{{cite journal | vauthors = Ngo HV, Martinetz T, Born J, Mölle M | title = Auditory closed-loop stimulation of the sleep slow oscillation enhances memory | journal = Neuron | volume = 78 | issue = 3 | pages = 545–53 | date = May 2013 | pmid = 23583623 | doi = 10.1016/j.neuron.2013.03.006 | doi-access = free }}</ref>

Reactivation of memory also occurs during wakefulness and its function is associated with serving to update the reactivated memory with newly encoded information, whereas reactivations during SWS are presented as crucial for memory stabilization.<ref name=":13" /> Based on targeted memory reactivation (TMR) experiments that use associated memory cues to triggering memory traces during sleep, several studies have been reassuring the importance of nocturnal reactivations for the formation of persistent memories in neocortical networks, as well as highlighting the possibility of increasing people's memory performance at declarative recalls.<ref name=":02" /><ref name=":22" /><ref name=":52" /><ref name=":32" /><ref>{{cite journal | vauthors = Klinzing JG, Kugler S, Soekadar SR, Rasch B, Born J, Diekelmann S | title = Odor cueing during slow-wave sleep benefits memory independently of low cholinergic tone | journal = Psychopharmacology | volume = 235 | issue = 1 | pages = 291–299 | date = January 2018 | pmid = 29119218 | pmc = 5748395 | doi = 10.1007/s00213-017-4768-5 }}</ref>

Furthermore, nocturnal reactivation seems to share the same neural oscillatory patterns as reactivation during wakefulness, processes which might be coordinated by [[Theta wave|theta activity]].<ref name=":4">{{cite journal | vauthors = Schreiner T, Doeller CF, Jensen O, Rasch B, Staudigl T | title = Theta Phase-Coordinated Memory Reactivation Reoccurs in a Slow-Oscillatory Rhythm during NREM Sleep | journal = Cell Reports | volume = 25 | issue = 2 | pages = 296–301 | date = October 2018 | pmid = 30304670 | pmc = 6198287 | doi = 10.1016/j.celrep.2018.09.037 }}</ref> During wakefulness, theta oscillations have been often related to successful performance in memory tasks, and cued memory reactivations during sleep have been showing that theta activity is significantly stronger in subsequent recognition of cued stimuli as compared to uncued ones, possibly indicating a strengthening of memory traces and lexical integration by cuing during sleep.<ref>{{cite journal | vauthors = Schreiner T, Göldi M, Rasch B | title = Cueing vocabulary during sleep increases theta activity during later recognition testing | journal = Psychophysiology | volume = 52 | issue = 11 | pages = 1538–43 | date = November 2015 | pmid = 26235609 | doi = 10.1111/psyp.12505 }}</ref> However, the beneficial effect of TMR for memory consolidation seems to occur only if the cued memories can be related to prior knowledge.<ref>{{cite journal | vauthors = Groch S, Schreiner T, Rasch B, Huber R, Wilhelm I | title = Prior knowledge is essential for the beneficial effect of targeted memory reactivation during sleep | journal = Scientific Reports | volume = 7 | pages = 39763 | date = January 2017 | pmid = 28051138 | pmc = 5209656 | doi = 10.1038/srep39763 | bibcode = 2017NatSR...739763G | doi-access = free }}</ref>

=== Dreaming ===
{{Main|Dream}}
[[File:Glimpse of a dream (9391068364).jpg|thumb|Dreams often feel like waking life, yet with added surrealism.]]During sleep, especially REM sleep, humans tend to experience dreams. These are elusive and mostly unpredictable first-person experiences which seem logical and realistic to the dreamer while they are in progress, despite their frequently bizarre, irrational, and/or surreal qualities that become apparent when assessed after waking. Dreams often seamlessly incorporate concepts, situations, people, and objects within a person's mind that would not normally go together. They can include apparent sensations of all types, especially vision and movement.<ref name=HobsonEtAl2000>J. Alan Hobson, Edward F. Pace-Scott, & Robert Stickgold (2000), "Dreaming and the brain: Toward a cognitive neuroscience of conscious states", ''Behavioral and Brain Sciences'' 23.</ref>

Dreams tend to rapidly fade from memory after waking. Some people choose to keep a [[dream journal]], which they believe helps them build dream recall and facilitate the ability to experience [[lucid dreams]].

A lucid dream is a type of dream in which the dreamer becomes aware that they are dreaming while dreaming. In a preliminary study, dreamers were able to consciously [[Lucid dream#Two-way communication|communicate]] with experimenters via eye movements or facial muscle signals, and were able to comprehend complex questions and use working memory.<ref name="10.1016/j.cub.2021.01.026">{{cite journal|display-authors=3 |last1=Konkoly |first1=Karen R. |last2=Appel |first2=Kristoffer |last3=Chabani |first3=Emma |last4=Mangiaruga |first4=Anastasia |last5=Gott |first5=Jarrod |last6=Mallett |first6=Remington |last7=Caughran |first7=Bruce |last8=Witkowski |first8=Sarah |last9=Whitmore |first9=Nathan W. |last10=Mazurek |first10=Christopher Y. |last11=Berent |first11=Jonathan B. |last12=Weber |first12=Frederik D. |last13=Türker |first13=Başak |last14=Leu-Semenescu |first14=Smaranda |last15=Maranci |first15=Jean-Baptiste |last16=Pipa |first16=Gordon |last17=Arnulf |first17=Isabelle |last18=Oudiette |first18=Delphine |last19=Dresler |first19=Martin |last20=Paller |first20=Ken A. |title=Real-time dialogue between experimenters and dreamers during REM sleep |journal=Current Biology |date=18 February 2021 |volume=31 |issue=7 |pages=1417–1427.e6 |doi=10.1016/j.cub.2021.01.026 |pmid=33607035 |pmc=8162929 |language=English |issn=0960-9822|doi-access=free |bibcode=2021CBio...31E1417K }} [[File:CC-BY icon.svg|50px]] Available under [https://creativecommons.org/licenses/by/4.0/ CC BY 4.0] {{Webarchive|url=https://web.archive.org/web/20171016050101/https://creativecommons.org/licenses/by/4.0/ |date=16 October 2017 }}.</ref>

People have proposed many [[hypotheses]] about the functions of dreaming. [[Sigmund Freud]] postulated that dreams are the symbolic expression of frustrated desires that have been relegated to the [[unconscious mind]], and he used [[dream interpretation]] in the form of [[psychoanalysis]] in attempting to uncover these desires.<ref>See Freud: ''[[The Interpretation of Dreams]]''.</ref>

Counterintuitively, [[nocturnal penile tumescence|penile erections during sleep]] are not more frequent during sexual dreams than during other dreams.<ref>{{cite book| vauthors = Pinel JP |title=Biopsychology, 8th Edition|year=2011|publisher=Pearson Education, Inc.|isbn=978-0-205-83256-9|page=359}}</ref> The [[parasympathetic nervous system]] experiences increased activity during REM sleep which may cause erection of the penis or clitoris. In males, 80% to 95% of REM sleep is normally accompanied by partial to full penile erection, while only about 12% of men's dreams contain sexual content.<ref name="Saladin 2012 537">{{cite book| vauthors = Saladin KS |title=Anatomy and Physiology: The Unity of Form and Function|edition=6th|year=2012|publisher=McGraw-Hill|isbn=978-0-07-337825-1|page=537}}</ref>