slow-wave sleep

{{Short description|Period of sleep in humans and other animals}}

{{redirect|Deep sleep|the video game series|Deep Sleep|the therapeutic treatment|Deep sleep therapy|the underground hotel|Deep Sleep (hotel)}}

File:Sleep Hypnogram.svg with slow-wave sleep level shown.]]

File:Ostriches-Sleep-like-Platypuses-pone.0023203.s003.ogves sleeping, with REM and slow-wave sleep phases{{cite journal | vauthors = Lesku JA, Meyer LC, Fuller A, Maloney SK, Dell'Omo G, Vyssotski AL, Rattenborg NC | title = Ostriches sleep like platypuses | journal = PLOS ONE | volume = 6 | issue = 8 | pages = e23203 | year = 2011 | pmid = 21887239 | pmc = 3160860 | doi = 10.1371/journal.pone.0023203 | veditors = Balaban E | doi-access = free | bibcode = 2011PLoSO...623203L }}]]

Slow-wave sleep (SWS), often referred to as deep sleep, is the third stage of non-rapid eye movement sleep (NREM), where electroencephalography activity is characterised by slow delta waves.{{cite book | vauthors = Rechtschaffen A, Kales A |title=A Manual of Standardized Terminology, Techniques and Scoring System for Sleep Stages of Human Subjects |publisher=US Dept of Health, Education, and Welfare; National Institutes of Health |year=1968}}

Slow-wave sleep usually lasts between 70 and 90 minutes, taking place during the first hours of the night.{{Cite journal |last=Reinoso Suárez |first=F. |date=1999 |title=[The neurobiology of slow-wave sleep] |url=https://pubmed.ncbi.nlm.nih.gov/10554397/ |journal=Anales de la Real Academia Nacional de Medicina |volume=116 |issue=1 |pages=209–224; discussion 224–226 |issn=0034-0634 |pmid=10554397}} Slow-wave sleep is characterised by moderate muscle tone, slow or absent eye movement, and lack of genital activity. Slow-wave sleep is considered important for memory consolidation, declarative memory, and the recovery of the brain from daily activities.

Before 2007, the term slow-wave sleep referred to the third and fourth stages of NREM. Current terminology combined these into a single stage three.{{cite book | vauthors = Carlson NR |title=Physiology of behavior |date=2013 |location=Boston |publisher=Pearson |isbn=978-0-205-23939-9 |edition=Eleventh }}{{rp|291}}

Overview

= Terminology =

This period of sleep is called slow-wave sleep because the EEG activity is synchronized, and characterised by slow waves with a frequency range of 0.5–4.5 Hz and a relatively high amplitude power with peak-to-peak amplitude greater than 75 μV. The first section of the wave signifies a "down state", an inhibition or hyperpolarizing phase in which the neurons in the neocortex are silent. This is the period when the neocortical neurons can rest. The second section of the wave signifies an "up state", an excitation or depolarizing phase in which the neurons fire briefly at a high rate. The principal characteristics during slow-wave sleep that contrast with REM sleep are moderate muscle tone, slow or absent eye movement, and lack of genital activity.{{rp|291, 293}}

Before 2007, the American Academy of Sleep Medicine (AASM) divided slow-wave sleep into stages 3 and 4.{{cite journal |vauthors=Schulz H |date=April 2008 |title=Rethinking sleep analysis |journal=Journal of Clinical Sleep Medicine |volume=4 |issue=2 |pages=99–103 |doi=10.5664/jcsm.27124 |pmc=2335403 |pmid=18468306 |quote=Although the sequence of non-REM (NREM) sleep stages one to four (R&K classification) or N1 to N3 (AASM classification) fulfills the criteria...}}{{cite web |year=2008 |title=Glossary. A resource from the Division of Sleep Medicine at Harvard Medical School, in partnership with WG Education Foundation |url=http://healthysleep.med.harvard.edu/glossary/n-p |url-status=live |archive-url=https://web.archive.org/web/20181004185342/http://healthysleep.med.harvard.edu/glossary/n-p |archive-date=2018-10-04 |access-date=2009-03-11 |publisher=Harvard University |quote=The 1968 categorization of the combined Sleep Stages 3 – 4 was reclassified in 2007 as Stage N3.}}Iber, C; Ancoli-Israel, S; Chesson, A; Quan, SF. for the American Academy of Sleep Medicine. The AASM Manual for the Scoring of Sleep and Associated Events: Rules, Terminology and Technical Specifications. Westchester: American Academy of Sleep Medicine; 2007. The two stages are now combined as Stage Three or N3. An epoch (30 seconds of sleep) that consists of 20% or more slow-wave (delta) sleep is now considered slow-wave sleep.

= Importance =

Slow-wave sleep is considered important for memory consolidation.{{Cite news|url = https://www.nytimes.com/2013/01/28/health/brain-aging-linked-to-sleep-related-memory-decline.html|title = Aging in Brain Found to Hurt Sleep Needed for Memory|newspaper = The New York Times|date = 2013-01-27|last1 = Carey|first1 = Benedict|access-date = 2017-04-17|archive-date = 2017-03-17|archive-url = https://web.archive.org/web/20170317104915/http://www.nytimes.com/2013/01/28/health/brain-aging-linked-to-sleep-related-memory-decline.html|url-status = live}} This is sometimes referred to as "sleep-dependent memory processing".{{cite journal | vauthors = Walker MP | title = Sleep-dependent memory processing | journal = Harvard Review of Psychiatry | volume = 16 | issue = 5 | pages = 287–98 | date = 1 January 2008 | pmid = 18803104 | doi = 10.1080/10673220802432517 }} Impaired memory consolidation has been seen in individuals with primary insomnia, who thus do not perform as well as those who are healthy in memory tasks following a period of sleep.{{Cite journal|last=Walker|first=Matthew P.|title=The Role of Slow Wave Sleep in Memory Processing|url=http://walkerlab.berkeley.edu/reprints/Walker_JCSM_2009.pdf|journal=Journal of Clinical Sleep Medicine|volume=5 |issue=2 Suppl |year=2009|doi=10.5664/jcsm.5.2S.S20 |pmid=19998871 |access-date=2014-05-06|archive-date=2013-05-09|archive-url=https://web.archive.org/web/20130509132319/http://walkerlab.berkeley.edu/reprints/Walker_JCSM_2009.pdf|url-status=live}}{{cite journal | vauthors = Walker MP | title = The role of slow wave sleep in memory processing | journal = Journal of Clinical Sleep Medicine | volume = 5 | issue = 2 Suppl | pages = S20-6 | date = April 2009 | pmid = 19998871 | pmc = 2824214 | doi = 10.5664/jcsm.5.2S.S20 }} Furthermore, slow-wave sleep improves declarative memory (which includes semantic and episodic memory). A central model has been hypothesized that long-term memory storage is facilitated by an interaction between the hippocampal and neocortical networks. In several studies, after the subjects have had the training to learn a declarative memory task, the density of human sleep spindles present was significantly higher than the signals observed during the control tasks, which involved similar visual stimulation and cognitively-demanding tasks but did not require learning.{{cite journal | vauthors = Steriade M | title = Grouping of brain rhythms in corticothalamic systems | journal = Neuroscience | volume = 137 | issue = 4 | pages = 1087–106 | date = 1 January 2006 | pmid = 16343791 | doi = 10.1016/j.neuroscience.2005.10.029 | url = https://pdfs.semanticscholar.org/672c/40468d7409acf579db860a0147088fdc933b.pdf | url-status = dead | s2cid = 15470045 | archive-url = https://web.archive.org/web/20170418083158/https://pdfs.semanticscholar.org/672c/40468d7409acf579db860a0147088fdc933b.pdf | archive-date = 18 April 2017 }}{{cite journal | vauthors = Gais S, Mölle M, Helms K, Born J | title = Learning-dependent increases in sleep spindle density | journal = The Journal of Neuroscience | volume = 22 | issue = 15 | pages = 6830–4 | date = August 2002 | pmid = 12151563 | pmc = 6758170 | doi = 10.1523/JNEUROSCI.22-15-06830.2002 | doi-access = free }} This associated with the spontaneously occurring wave oscillations that account for the intracellular recordings from thalamic and cortical neurons.{{Cite journal|last=Steriade|first=M.|date=2004|title=Slow-wave sleep: serotonin, neuronal plasticity, and seizures|url=http://www.architalbiol.org/aib/article/viewFile/411/370|url-status=dead|journal=Archives Italienne de Biologie|volume=142|issue=4|pages=359–367|pmid=15493541|archive-url=https://web.archive.org/web/20140506212855/http://www.architalbiol.org/aib/article/viewFile/411/370|archive-date=2014-05-06|access-date=2014-05-06}}

Specifically, SWS presents a role in spatial declarative memory. Reactivation of the hippocampus during SWS is detected after the spatial learning task.{{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 }} In addition, a correlation can be observed between the amplitude of hippocampal activity during SWS and the improvement in spatial memory performance, such as route retrieval, on the following day.{{cite journal | vauthors = Peigneux P, Laureys S, Fuchs S, Collette F, Perrin F, Reggers J, Phillips C, Degueldre C, Del Fiore G, Aerts J, Luxen A, Maquet P | display-authors = 6 | title = Are spatial memories strengthened in the human hippocampus during slow wave sleep? | journal = Neuron | volume = 44 | issue = 3 | pages = 535–45 | date = October 2004 | pmid = 15504332 | doi = 10.1016/j.neuron.2004.10.007 | s2cid = 1424898 | hdl = 2268/21205 | hdl-access = free }} Additionally, studies have found that when odour cues are given to subjects during sleep, this stage of sleep exclusively allows contextual cues to be reactivated after sleep, favoring their consolidation. A separate study found that when subjects hear sounds associated with previously shown pictures of locations, the reactivation of individual memory representations was significantly higher during SWS as compared to other sleep stages.{{cite journal | vauthors = Cairney SA, Durrant SJ, Hulleman J, Lewis PA | title = Targeted memory reactivation during slow wave sleep facilitates emotional memory consolidation | journal = Sleep | volume = 37 | issue = 4 | pages = 701–7, 707A | date = April 2014 | pmid = 24688163 | pmc = 3954173 | doi = 10.5665/sleep.3572 }}

Affective representations are generally better remembered during sleep compared to neutral ones. Emotions with negative salience presented as a cue during SWS show better reactivation, and therefore an enhanced consolidation in comparison to neutral memories. The former was predicted by sleep spindles over SWS, which discriminates the memory processes during sleep as well as facilitating emotional memory consolidation. Acetylcholine plays an essential role in hippocampus-dependent memory consolidation. An increased level of cholinergic activity during SWS is known to be disruptive to memory processing. Considering that acetylcholine is a neurotransmitter that modulates the direction of information flow between the hippocampus and neocortex during sleep, its suppression is necessary during SWS to consolidate sleep-related declarative memory.{{cite journal | vauthors = Gais S, Born J | title = Low acetylcholine during slow-wave sleep is critical for declarative memory consolidation | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 101 | issue = 7 | pages = 2140–4 | date = February 2004 | pmid = 14766981 | pmc = 357065 | doi = 10.1073/pnas.0305404101 | doi-access = free | bibcode = 2004PNAS..101.2140G }}

Sleep deprivation studies with humans suggest that the primary function of slow-wave sleep may be to allow the brain to recover from its daily activities. Glucose metabolism in the brain increases as a result of tasks that demand mental activity. Another function affected by slow-wave sleep is the secretion of growth hormone, which is always greatest during this stage.{{cite book|title=Slow-Wave Sleep: Beyond Insomnia|publisher=Wolters Kluwer Pharma Solutions|isbn=978-0-9561387-1-2}} It is also thought to be responsible for a decrease in sympathetic and increase in parasympathetic neural activity.

Electroencephalographic characteristics

File:Sleep EEG Stage 4.jpg

Large 75-microvolt (0.5–2.0 Hz) delta waves predominate the electroencephalogram (EEG). Stage N3 is defined by the presence of 20% delta waves in any given 30-second epoch of the EEG during sleep, by the current 2007 AASM guidelines.{{cite journal | vauthors = Brancaccio A, Tabarelli D, Bigica M, Baldauf D | title = Cortical source localization of sleep-stage specific oscillatory activity | journal = Scientific Reports | volume = 10 | issue = 1 | pages = 6976 | date = April 2020 | pmid = 32332806 | pmc = 7181624 | doi = 10.1038/s41598-020-63933-5 | bibcode = 2020NatSR..10.6976B }}

Longer periods of SWS occur in the first part of the night, primarily in the first two sleep cycles (roughly three hours). Children and young adults will have more total SWS in a night than older adults. The elderly may not go into SWS at all during many nights of sleep.{{citation needed|date=May 2023}}

NREM sleep, as observed on the electroencephalogram (EEG), is distinguished by certain characteristic features. Sleep spindles, marked by spindle-like changes in the amplitude of 12–14 Hz oscillations, K complexes lasting at least 0.5 seconds, consisting of a distinct negative sharp wave followed by a positive component, and slow waves or delta waves characterized by slow frequency (< 2 Hz) and high amplitude (> 75 μV) are key indicators.{{cite journal |last1=Dijk |first1=Derk-Jan |author-link1=Derk-Jan Dijk |date=2009 |title=Regulation and Functional Correlates of Slow Wave Sleep |journal=J Clin Sleep Med |volume=5 |issue=2 Suppl |pages=6–15 |doi=10.5664/jcsm.5.2S.S6 |pmid=19998869 |pmc=2824213}} The presence and distribution of sleep spindle activity and slow waves vary across NREM sleep, leading to its subdivision into stages 1–3. While slow waves and sleep spindles are present in stages 2 and 3, stage 2 sleep is characterized by a higher prevalence of spindles, while slow waves dominate the EEG during stage 3.{{cite journal |last1=Dijk |first1=D. J. |author-link1=Derk-Jan Dijk |last2=Hayes |first2=B. |last3=Czeisler |first3=C. A. |date=1993 |title=Dynamics of electroencephalographic sleep spindles and slow wave activity in men: effect of sleep deprivation |url=https://pubmed.ncbi.nlm.nih.gov/8281430/ |journal=Brain Res. |volume=626 |issue=1–2 |pages=190–9 |doi=10.1016/0006-8993(93)90579-c |pmid=8281430|s2cid=42683788 }}

Slow-wave sleep is an active phenomenon probably brought about by the activation of serotonergic neurons of the raphe system.{{cite journal | vauthors = Jones BE | title = Arousal systems | journal = Frontiers in Bioscience | volume = 8 | issue = 6 | pages = s438-51 | date = May 2003 | pmid = 12700104 | doi = 10.2741/1074 | doi-access = free }}

The slow wave seen in the cortical EEG is generated through recurrent connections within the cerebral cortex, where cortical pyramidal cells excite one another in a positive feedback loop. This recurrent excitation is balanced by inhibition, resulting in the active state of the slow oscillation of slow wave sleep. Failure of this mechanism results in a silencing of activity for a brief period. The recurrence of active and silent periods occurs at a rate of 0.5–4 Hz, giving rise to the slow waves of the EEG seen during slow wave sleep.{{cite journal |last1=Sanchez-Vives |first1=MV |last2=McCormick |first2=DA |title=Cellular and network mechanisms of rhythmic recurrent activity in neocortex |journal=Nat Neuroscience |date=2000 |volume=3 |issue=10 |pages=1027–1034 |doi=10.1038/79848|pmid=11017176 |s2cid=509469 }}

Functions

=Hemispheric asymmetries in the human sleep=

Slow-wave sleep is necessary for survival. Some animals, such as dolphins and birds, can sleep with only one hemisphere of the brain, leaving the other hemisphere awake to carry out normal functions and to remain alert. This kind of sleep is called unihemispheric slow-wave sleep, and is also partially observable in humans. Indeed, a study reported a unilateral activation of the somatosensorial cortex when a vibrating stimulus was put on the hand of human subjects. The recordings show an important inter-hemispheric change during the first hour of non-REM sleep and consequently the presence of a local and use-dependent aspect of sleep.{{cite journal | vauthors = Kattler H, Dijk DJ, Borbély AA | title = Effect of unilateral somatosensory stimulation before sleep on the sleep EEG in humans | journal = Journal of Sleep Research | volume = 3 | issue = 3 | pages = 159–164 | date = September 1994 | pmid = 10607121 | doi = 10.1111/j.1365-2869.1994.tb00123.x | s2cid = 26078900 | doi-access = free }} Another experiment detected a greater number of delta waves in the frontal and central regions of the right hemisphere.{{cite journal | vauthors = Sekimoto M, Kato M, Kajimura N, Watanabe T, Takahashi K, Okuma T | title = Asymmetric interhemispheric delta waves during all-night sleep in humans | journal = Clinical Neurophysiology | volume = 111 | issue = 5 | pages = 924–8 | date = May 2000 | pmid = 10802465 | doi = 10.1016/S1388-2457(00)00258-3 | s2cid = 44808363 }}

Considering that SWS is the only sleep stage that reports human deep sleep as well as being used in studies with mammals and birds, it is also adopted in experiments revealing the role of hemispheric asymmetries during sleep. A predominance of the left hemisphere in the neural activity can be observed in the default-mode network during SWS. This asymmetry is correlated with the sleep onset latency, which is a sensitive parameter of the so-called first night effect—the reduced quality of sleep during the first session in the laboratory.{{cite journal | vauthors = Tamaki M, Bang JW, Watanabe T, Sasaki Y | title = Night Watch in One Brain Hemisphere during Sleep Associated with the First-Night Effect in Humans | journal = Current Biology | volume = 26 | issue = 9 | pages = 1190–4 | date = May 2016 | pmid = 27112296 | pmc = 4864126 | doi = 10.1016/j.cub.2016.02.063 | bibcode = 2016CBio...26.1190T }}

The left hemisphere is shown to be more sensitive to deviant stimuli during the first night—compared to the following nights of an experiment. This asymmetry explains further the reduced sleep of half the brain during SWS. Indeed, in comparison to the right one, the left hemisphere plays a vigilant role during SWS.

Furthermore, a faster behavioral reactivity is detected in the left hemisphere during SWS of the first night. The rapid awakening is correlated to the regional asymmetry in the activities of SWS. These findings show that the hemispheric asymmetry in SWS plays a role as a protective mechanism. SWS is therefore sensitive to danger and a non-familiar environment, creating a need for vigilance and reactivity during sleep.

=Neural control of slow-wave sleep=

Several neurotransmitters are involved in sleep and waking patterns: acetylcholine, norepinephrine, serotonin, histamine, and orexin.{{rp|305–307}} Neocortical neurons fire spontaneously during slow-wave sleep, thus they seem to play a role during this period of sleep. Also, these neurons appear to have some form of internal dialogue, which accounts for the mental activity during this state where there is no information from external signals (because of the synaptic inhibition at the thalamic level). The rate of recall of dreams during this state of sleep is relatively high compared to the other levels of the sleep cycle. This indicates that mental activity is closer to real-life events.

=Physical healing and growth=

Slow-wave sleep is the constructive phase of sleep for recuperation of the mind-body system in which it rebuilds itself after each day. Substances that have been ingested into the body while an organism is awake are synthesized into complex proteins of living tissue. Growth hormone is also secreted during this stage, which leads some scientists to hypothesize that a function of slow-wave sleep is to facilitate the healing of muscles as well as repair damage to tissues.{{Cite web|date=2009-12-22|title=What Happens When You Sleep: The Science of Sleep|url=https://www.sleepfoundation.org/how-sleep-works/what-happens-when-you-sleep|access-date=2021-06-25|website=Sleep Foundation|language=en|archive-date=2021-06-21|archive-url=https://web.archive.org/web/20210621113251/https://www.sleepfoundation.org/how-sleep-works/what-happens-when-you-sleep|url-status=live}}{{Cite journal|vauthors=Payne JD, Walker WP|date=June 2008|title=Review article: Does delta sleep matter?|url=https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.723.1235&rep=rep1&type=pdf|journal=INSOM: INSomnia and Its Optimized Management|issue=10|pages=3–6|citeseerx=10.1.1.723.1235|via=CiteSeerX|access-date=2021-06-25|archive-date=2021-06-25|archive-url=https://web.archive.org/web/20210625225808/https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.723.1235&rep=rep1&type=pdf|url-status=live}} Lastly, glial cells within the brain are restored with sugars to provide energy for the brain.{{Cite web|date=2015-10-23|title=The importance of sleep & why we need it|url=http://www.hgi.org.uk/archive/sleepanddream1.htm|url-status=live|archive-url=https://web.archive.org/web/20190711122951/https://www.hgi.org.uk/resources/delve-our-extensive-library/dreaming/importance-sleep-why-we-need-it|archive-date=July 11, 2019|website=Human Givens Institute}}

=Learning and synaptic homeostasis=

Learning and memory formation occur during wakefulness by the process of long-term potentiation; SWS is associated with the regulation of synapses thus potentiated. SWS is involved in the downscaling of synapses, in which strongly stimulated or potentiated synapses are kept while weakly potentiated synapses either diminish or are removed.{{cite journal | vauthors = Tononi G, Cirelli C | title = Sleep function and synaptic homeostasis | journal = Sleep Medicine Reviews | volume = 10 | issue = 1 | pages = 49–62 | date = February 2006 | pmid = 16376591 | doi = 10.1016/j.smrv.2005.05.002 | s2cid = 16129740 }} This may be helpful for recalibrating synapses for the next potentiation during wakefulness and for maintaining synaptic plasticity. Notably, new evidence is showing that reactivation and rescaling may be co-occurring during sleep.{{cite journal | vauthors = Gulati T, Guo L, Ramanathan DS, Bodepudi A, Ganguly K | title = Neural reactivations during sleep determine network credit assignment | journal = Nature Neuroscience | volume = 20 | issue = 9 | pages = 1277–1284 | date = September 2017 | pmid = 28692062 | pmc = 5808917 | doi = 10.1038/nn.4601 }}

Problems associated with slow-wave sleep

Bedwetting, night terrors, and sleepwalking are all common behaviors that can occur during stage three of sleep. These occur most frequently amongst children, who generally outgrow them.{{rp|297–8}} Another problem that may arise is sleep-related eating disorder. An individual will sleep-walk seeking out food, and will eat not having any memory of the event in the morning. Over half of individuals with this disorder become overweight.{{rp|298}} Sleep-related eating disorder can usually be treated with dopaminergic agonists, or topiramate, which is an anti-seizure medication. Heredity may be factor with this disorder.

=Effects of sleep deprivation=

J. A. Horne (1978) reviewed several experiments with humans and concluded that sleep deprivation has no effects on people's physiological stress response or ability to perform physical exercise. It did, however, have an effect on cognitive functions. Some people reported distorted perceptions or hallucinations and a lack of concentration on mental tasks. Thus, the major role of sleep does not appear to be rest for the body but rest for the brain.

When sleep-deprived humans sleep normally again, the recovery percentage for each stage of sleep is not the same. Only seven percent of stages one and two are regained, but 68 percent of stage-four slow-wave sleep and 53 percent of REM sleep are regained. This suggests that stage-four sleep (known today as the deepest part of stage-three sleep) is more important than the other stages.

During slow-wave sleep, there is a significant decline in cerebral metabolic rate and cerebral blood flow. The activity falls to about 75 percent of the normal wakefulness level. The regions of the brain that are most active when awake have the highest level of delta waves during slow-wave sleep. This indicates that the rest is geographical. The "shutting down" of the brain accounts for the grogginess and confusion if someone is awakened during deep sleep since it takes the cerebral cortex time to resume its normal functions.

According to J. Siegel (2005), sleep deprivation results in the build-up of free radicals and superoxides in the brain. Free radicals are oxidizing agents that have one unpaired electron, making them highly reactive. These free radicals interact with electrons of biomolecules and damage cells. In slow-wave sleep, the decreased rate of metabolism reduces the creation of oxygen byproducts, thereby allowing the existing radical species to clear. This is a means of preventing damage to the brain.Carlson, Neil R. (2012). Physiology of Behavior. Pearson. p. 299-300. {{ISBN|0205239390}}.

=Amyloid beta pathology=

Results from a number of research have shown how sleep affects Aβ dynamics.{{cite journal |last1=Varga |first1= Andrew W. |last2=Wohlleber |first2=Margaret E. |last3=Giménez | first3=Sandra | last4=Romero |first4= Sergio |last5= Alonso |first5=Joan F. |last6=Ducca |first6= Emma L. |last7= Kam |first7= Korey |last8=Lewis |first8=Clifton |last9=Tanzi |first9=Emily B. |last10=Tweardy |first10=Samuel |last11=Kishi |first11=Akifumi |last12=Parekh |first12=Ankit |last13=Fischer |first13=Esther |last14=Gumb |first14=Tyler |last15=Alcolea |first15=Daniel |last16=Fortea |first16=Juan |last17=Lleó |first17=Alberto |last18=Blennow |first18=Kaj |last19=Zetterberg |first19=Henrik |last20=Mosconi |first20=Lisa |last21=Glodzik |first21=Lidia |last22=Pirraglia |first22=Elizabeth |last23=Burschtin |first23=Omar |last24=Leon |first24=Mony J. |last25=Rapoport |first25=David M. |last26=Lu |first26=Shou-en Lu |last27=Ayappa |first27=Indu |last28=Osorio |first28=Ricardo S. |date=2016 |title=Reduced Slow-Wave Sleep Is Associated with High Cerebrospinal Fluid Aβ42 Levels in Cognitively Normal Elderly |journal=Sleep | volume = 39 | issue = 11 | pages = 2041–2048 |doi =10.5665/sleep.6240 | pmid=27568802|pmc=5070758}} A good candidate for slow wave activity, which occurs during deep non-REM sleep, is amyloid-b modulation. The researchers also highlighted a strong relationship between amyloid-b and SWA, pointing out that increased disruption in SWA is correlated with elevated levels of amyloid-b.{{cite journal |last1=Ju |first1= Yo-El |last2=Ooms |first2=Sharon J. |last3=Sutphen | first3=Courtney | last4=Macauley |first4= Shannon L. |last5= Zangrilli |first5=Margaret A. |last6=Jerome |first6= Gina |last7= Fagan |first7= Anne M. |last8=Mignot |first8=Emmanuel |last9=Zempel |first9=John M. |last10=Claassen |first10=Jurgen A. |last11=Holtzman |first11=David|date=2017 |title=Slow wave sleep disruption increases cerebrospinal fluid amyloid-β levels |journal=Brain |volume=140 |issue=8 |pages=2104–2111 |doi=10.1093/brain/awx148 |pmid=28899014 |pmc=5790144}} Hence, Slow waves of non-rapid eye movement sleep are disrupted or decrease when amyloid beta (Aβ) builds up in the prefrontal cortex. As a result, this may hinder older people's capacity for memory consolidation.{{cite journal | vauthors = Mander BA, Marks SM, Vogel JW, Rao V, Lu B, Saletin JM, Ancoli-Israel S, Jagust WJ, Walker MP | display-authors = 6 | title = β-amyloid disrupts human NREM slow waves and related hippocampus-dependent memory consolidation | journal = Nature Neuroscience | volume = 18 | issue = 7 | pages = 1051–7 | date = July 2015 | pmid = 26030850 | pmc = 4482795 | doi = 10.1038/nn.4035 }}

Moreover, the onset of Alzheimer's disease is marked by the deposition of amyloid beta (Aβ) in the brain. AD is distinguished by the presence of amyloid-beta plaques and neurofibrillary tangles. These structural anomalies are linked to disruptions in the sleep-wake cycle, particularly in non-REM slow wave sleep.{{cite journal |last1=Lee |first1= Lee Fun |last2=Gerashchenko |first2=Dmitry |last3=Timofeev | first3=Igor | last4=Bacskai |first4=Brian J. |last5= Kastanenka |first5=Ksenia V. |date=2020 |title=Slow Wave Sleep Is a Promising Intervention Target for Alzheimer's Disease |journal=Front. Neurosci. |volume=14 |pages=705 |doi=10.3389/fnins.2020.00705 |doi-access= free |pmid=32714142 |pmc=7340158}} Thus, individuals diagnosed with Alzheimer's often experience disturbances in sleep, resulting in diminished levels of non-rapid eye movement sleep and reduced slow wave activity, that is a prominent brain rhythm during deep non-REM sleep.{{cite journal |last1=Moran |first1=Maria |last2=Lynch |first2=C. A. |last3=Walsh | first3=C. | last4=Coen |first4=R. |last5=Coakley |first5=D. |last6=Lawlor |first6=B. A. |date=2005 |title=Sleep disturbance in mild to moderate Alzheimer's disease |url=https://pubmed.ncbi.nlm.nih.gov/15978517/ |journal=Sleep Med. |volume=6 |issue=4 |pages=347–52 |doi=10.1016/j.sleep.2004.12.005 |pmid=15978517}} Similarly, even cognitively healthy individuals with detectable amyloid beta exhibit sleep disturbances, characterized by compromised sleep quality and an increased frequency of daytime napping.

Individual differences

Though slow-wave sleep is fairly consistent within the individual, it can vary across individuals.{{cite journal | vauthors = Mokhlesi B, Pannain S, Ghods F, Knutson KL | title = Predictors of slow-wave sleep in a clinic-based sample | journal = Journal of Sleep Research | volume = 21 | issue = 2 | pages = 170–5 | date = April 2012 | pmid = 21955220 | pmc = 3321544 | doi = 10.1111/j.1365-2869.2011.00959.x | author-link4 = Kristen L Knutson }} Individual variations seem to be influenced by demographic factors such as gender and age.

Slow-wave sleep and slow-wave activity undergo significant transformations throughout one's lifespan, with aging serving as a particularly influential factor in predicting individual variations.{{cite journal |last1=Dijk |first1=Derk-Jan |author-link1=Derk-Jan Dijk |last2=Groeger |first2=John A. |last3=Stanley |first3=Neil |last4=Deacon |first4=Stephen |date=2010 |title=Age-related reduction in daytime sleep propensity and nocturnal slow wave sleep |journal=Sleep |volume=33 |issue=2 |pages=211–23 |doi=10.1093/sleep/33.2.211 |pmid=20175405 |pmc=2817908}} Aging is inversely proportional to the amount of SWS beginning by midlife, so slow-wave sleep declines with age. Moreover, recent findings indicate that older individuals exhibit a decreased inclination for daytime sleep compared to younger counterparts, and this decline persists even when accounting for variations in habitual sleep duration. This age-related decrease in daytime sleep propensity is evident in middle-aged individuals and coincides with statistically significant reductions in total sleep time, slow-wave sleep, and slow-wave activity.

Sex differences have also been found, such that females tend to have higher levels of slow-wave sleep than males, at least up until menopause. Older individuals exhibit gender-based variations in non-rapid eye movement (NREM) sleep, where women demonstrate increased slow-wave sleep during both regular and recuperative sleep.

There have also been studies that have shown differences between races. The results showed that there was a lower percentage of slow-wave sleep in African Americans compared to Caucasians, but since there are many influencing factors (e.g., body mass index, sleep-disordered breathing, obesity, diabetes, and hypertension), this potential difference must be investigated further.

Mental disorders play a role in individual differences in the quality and quantity of slow-wave sleep: subjects with depression show a lower amplitude of slow-wave activity compared to healthy participants. Sex differences also persist in the former group: depressed men present significantly lower slow-wave amplitude. This sex divergence is twice as large as the one observed in healthy subjects. However, no age-related difference concerning slow-wave sleep can be observed in the depressed group.{{cite journal |vauthors=Armitage R, Hoffmann R, Trivedi M, Rush AJ |date=September 2000 |title=Slow-wave activity in NREM sleep: sex and age effects in depressed outpatients and healthy controls |url=https://deepblue.lib.umich.edu/bitstream/handle/2027.42/60178/Armitage%2c%20Hoffmann%2c%20Triveidi%2c%20Rush%202000.pdf?sequence=2&isAllowed=y |journal=Psychiatry Research |volume=95 |issue=3 |pages=201–13 |doi=10.1016/S0165-1781(00)00178-5 |pmid=10974359 |s2cid=1903649}}

Brain regions

During sleep, the distribution of slow-wave activity (SWA) typically exhibits a prevalence in the frontal region of the brain.{{cite journal |last1=Finelli |first1=L.A. |last2= Borbély |first2=A.A. |last3=Achermann |first3=P. |date=2001 |title=Functional topography of the human nonREM sleep electroencephalogram |url=https://pubmed.ncbi.nlm.nih.gov/11454032/ |journal=Eur J Neurosci |volume=13 |issue=12 |pages=2282–90 |doi= 10.1046/j.0953-816x.2001.01597.x |pmid=11454032|s2cid=206682 }} In the subsequent recovery sleep after experiencing sleep deprivation, the frontal cortex exhibits the most significant rise in slow-wave activity (SWA) compared to the temporal region, parietal region, and occipital region. The notable increase in SWA following sleep deprivation in the frontal areas, coupled with the prevailing presence of SWA in the frontal regions even during baseline sleep, has been construed as evidence supporting the involvement of slow-wave sleep (SWS) in functions typically linked to the frontal cortices. Thus, the prevalence of slow-wave sleep (SWS) in the frontal regions, particularly those linked to advanced cognitive functions or cognitive regions highly active during wakefulness, underscores the considerable importance of SWS.

Some of the brain regions implicated in the induction of slow-wave sleep include:

The parafacial zone (GABAergic neurons),{{cite journal | vauthors = Anaclet C, Ferrari L, Arrigoni E, Bass CE, Saper CB, Lu J, Fuller PM | title = The GABAergic parafacial zone is a medullary slow wave sleep-promoting center | journal = Nat. Neurosci. | volume = 17 | issue = 9 | pages = 1217–1224 | date = September 2014 | pmid = 25129078 | pmc = 4214681 | doi = 10.1038/nn.3789 | quote = In the present study we show, for the first time, that activation of a delimited node of GABAergic neurons located in the medullary PZ can potently initiate SWS and cortical SWA in behaving animals. ... For now however it remains unclear if the PZ is interconnected with other sleep– and wake–promoting nodes beyond the wake–promoting PB. ... The intensity of cortical slow–wave–activity (SWA: 0.5–4Hz) during SWS is also widely accepted as a reliable indicator of sleep need ... In conclusion, in the present study we demonstrated that all polygraphic and neurobehavioral manifestation of SWS, including SWA, can be initiated in behaving animals by the selective activation of a delimited node of GABAergic medullary neurons. | url = https://dash.harvard.edu/bitstream/handle/1/14351306/4214681.pdf?sequence=1 | access-date = 2018-11-04 | archive-date = 2018-11-04 | archive-url = https://web.archive.org/web/20181104211018/https://dash.harvard.edu/bitstream/handle/1/14351306/4214681.pdf?sequence=1 | url-status = live }}{{cite journal | vauthors = Schwartz MD, Kilduff TS | title = The Neurobiology of Sleep and Wakefulness | journal = The Psychiatric Clinics of North America | volume = 38 | issue = 4 | pages = 615–644 | date = December 2015 | pmid = 26600100 | pmc = 4660253 | doi = 10.1016/j.psc.2015.07.002 | quote = More recently, the medullary parafacial zone (PZ) adjacent to the facial nerve was identified as a sleep-promoting center on the basis of anatomical, electrophysiological and chemo- and optogenetic studies.^{23, 24} GABAergic PZ neurons inhibit glutamatergic parabrachial (PB) neurons that project to the BF,²⁵ thereby promoting NREM sleep at the expense of wakefulness and REM sleep. ... Sleep is regulated by GABAergic populations in both the preoptic area and the brainstem; increasing evidence suggests a role for the melanin-concentrating hormone cells of the lateral hypothalamus and the parafacial zone of the brainstem}}{{cite journal | vauthors = Brown RE, McKenna JT | title = Turning a Negative into a Positive: Ascending GABAergic Control of Cortical Activation and Arousal | journal = Front. Neurol. | volume = 6 | pages = 135 | date = June 2015 | pmid = 26124745 | pmc = 4463930 | doi = 10.3389/fneur.2015.00135 | quote = The sleep-promoting action of GABAergic neurons located in the preoptic hypothalamus (6–8) is now well-known and accepted (9). More recently, other groups of sleep-promoting GABAergic neurons in the lateral hypothalamus (melanin-concentrating hormone neurons) and brainstem [parafacial zone; (10)] have been identified.| doi-access = free }} located within the medulla oblongata
the nucleus accumbens core (GABAergic medium spiny neurons; specifically, the subset of these neurons that expresses both D2-type dopamine receptors and adenosine A_2A receptors),{{cite journal | vauthors = Valencia Garcia S, Fort P | title = Nucleus Accumbens, a new sleep-regulating area through the integration of motivational stimuli | journal = Acta Pharmacologica Sinica | volume = 39| issue = 2| pages = 165–166| date = December 2017 | pmid = 29283174 | pmc = 5800466 | doi = 10.1038/aps.2017.168 | quote = The nucleus accumbens comprises a contingent of neurons specifically expressing the post-synaptic A2A-receptor (A2AR) subtype making them excitable by adenosine, its natural agonist endowed with powerful sleep-promoting properties[4]. ... In both cases, large activation of A2AR-expressing neurons in NAc promotes slow wave sleep (SWS) by increasing the number and duration of episodes. ... After optogenetic activation of the core, a similar promotion of SWS was observed, whereas no significant effects were induced when activating A2AR-expressing neurons within the shell.}}{{cite journal | vauthors = Oishi Y, Xu Q, Wang L, Zhang BJ, Takahashi K, Takata Y, Luo YJ, Cherasse Y, Schiffmann SN, de Kerchove d'Exaerde A, Urade Y, Qu WM, Huang ZL, Lazarus M | title = Slow-wave sleep is controlled by a subset of nucleus accumbens core neurons in mice | journal = Nature Communications | volume = 8 | issue = 1 | pages = 734 | date = September 2017 | pmid = 28963505 | pmc = 5622037 | doi = 10.1038/s41467-017-00781-4 | quote = Here, we show that chemogenetic or optogenetic activation of excitatory adenosine A2A receptor-expressing indirect pathway neurons in the core region of the NAc strongly induces slow-wave sleep. Chemogenetic inhibition of the NAc indirect pathway neurons prevents the sleep induction, but does not affect the homoeostatic sleep rebound.| bibcode = 2017NatCo...8..734O }}{{cite journal | vauthors = Yuan XS, Wang L, Dong H, Qu WM, Yang SR, Cherasse Y, Lazarus M, Schiffmann SN, d'Exaerde AK, Li RX, Huang ZL | title = Striatal adenosine A2A receptor neurons control active-period sleep via parvalbumin neurons in external globus pallidus | journal = eLife | volume = 6 | pages = e29055 | date = October 2017 | pmid = 29022877 | pmc = 5655138 | doi = 10.7554/eLife.29055 | doi-access = free }} located within the striatum
the ventrolateral preoptic area (GABAergic neurons),{{cite journal | vauthors = Varin C, Rancillac A, Geoffroy H, Arthaud S, Fort P, Gallopin T | title = Glucose Induces Slow-Wave Sleep by Exciting the Sleep-Promoting Neurons in the Ventrolateral Preoptic Nucleus: A New Link between Sleep and Metabolism | journal = The Journal of Neuroscience | volume = 35 | issue = 27 | pages = 9900–11 | year = 2015 | pmid = 26156991 | pmc = 6605416 | doi = 10.1523/JNEUROSCI.0609-15.2015 }} located within the hypothalamus
The lateral hypothalamus (melanin-concentrating hormone-releasing neurons),{{cite journal | vauthors = Monti JM, Torterolo P, Lagos P | title = Melanin-concentrating hormone control of sleep-wake behavior | journal = Sleep Medicine Reviews | volume = 17 | issue = 4 | pages = 293–8 | year = 2013 | pmid = 23477948 | doi = 10.1016/j.smrv.2012.10.002 | quote = MCHergic neurons are silent during wakefulness (W), increase their firing during slow wave sleep (SWS) and still more during REM sleep (REMS). Studies in knockout mice for MCH (MCH(-/-)) have shown a reduction in SWS and an increase of W during the light and the dark phase of the light-dark cycle.}}{{cite journal | vauthors = Torterolo P, Lagos P, Monti JM | title = Melanin-concentrating hormone: a new sleep factor? | journal = Frontiers in Neurology | volume = 2 | pages = 14 | year = 2011 | pmid = 21516258 | pmc = 3080035 | doi = 10.3389/fneur.2011.00014 | quote = Neurons containing the neuropeptide melanin-concentrating hormone (MCH) are mainly located in the lateral hypothalamus and the incerto-hypothalamic area, and have widespread projections throughout the brain. ... Intraventricular microinjection of MCH increases both slow wave sleep (SWS) and REM sleep; however, the increment in REM sleep is more pronounced. ... Although both SWS and REM sleep are facilitated by MCH, REM sleep seems to be more sensitive to MCH modulation.| doi-access = free }} located within the hypothalamus

Drugs

Some drugs influence sleep architecture by encroaching upon or prolonging deep sleep.{{cite journal |last1=Roehrs |first1=Timothy |last2=Roth |first2=Thomas |date=2011 |title=Drug-related Sleep Stage Changes: Functional Significance and Clinical Relevance |journal=Sleep Med Clin. |volume=5 |issue=4 |pages=559–570 |doi=10.1016/j.jsmc.2010.08.002 |pmid= 21344068 |pmc=3041980}} Many drugs known to increase deep sleep in humans are of the GABAergic, dopaminergic, and anti-serotonergic classes.{{cite journal |last1=Walsh |first1=James K.|date=2009 |title=Enhancement of Slow Wave Sleep: Implications for Insomnia |journal=J Clin Sleep Med |volume=5 |issue=2 Suppl |pages=27–32 |doi=10.5664/jcsm.5.2S.S27 |pmid=19998872 |pmc=2824211}}

Gamma-hydroxybutyrate is synthesized in the central nervous system from gamma-aminobutyric acid (GABA). Oral administration has been shown to enhance slow-wave sleep without suppressing REM sleep.{{Cite journal |last1=Mamelak |first1=M. |last2=Escriu |first2=J. M. |last3=Stokan |first3=O. |date=April 1977 |title=The effects of gamma-hydroxybutyrate on sleep |url=https://pubmed.ncbi.nlm.nih.gov/192353/ |journal=Biological Psychiatry |volume=12 |issue=2 |pages=273–288 |issn=0006-3223 |pmid=192353}}{{cite journal | vauthors = Roehrs T, Roth T | title = Drug-related Sleep Stage Changes: Functional Significance and Clinical Relevance | journal = Sleep Medicine Clinics | volume = 5 | issue = 4 | pages = 559–570 | date = December 2010 | pmid = 21344068 | pmc = 3041980 | doi = 10.1016/j.jsmc.2010.08.002 }}{{Cite web | url=http://www.theodora.com/drugs/eu/xyrem.html | title=Xyrem - European Drugs Reference Encyclopedia | access-date=2013-07-16 | archive-date=2013-08-21 | archive-url=https://web.archive.org/web/20130821022418/http://www.theodora.com/drugs/eu/xyrem.html | url-status=live }} In the United States, it is sold as a prescription drug under the brand name Xyrem. It has been shown to reduce cataplexy attacks and excessive daytime sleepiness in patients with narcolepsy.

The administration of the GABA_a agonist gabaxadol enhances both deep sleep and also positively impacts various indicators of insomnia.

Tiagabine, a selective GABA reuptake inhibitor, demonstrated improving sleep maintenance and significantly increasing SWS in healthy elderly subjects and adult patients with primary insomnia.{{cite journal |last1=Walsh |first1=James K. |last2=Zammit |first2=Gary |last3=Schweitzer |first3=Paula K. |last4=Ondrasik |first4=John |last5=Roth |first5=Thomas |date=2005 |title=Tiagabine enhances slow wave sleep and sleep maintenance in primary insomnia |url=https://pubmed.ncbi.nlm.nih.gov/16260179/ |journal=Sleep Med. |volume=7 |issue=2 |pages=155–61 |doi= 10.1016/j.sleep.2005.05.004 |pmid=16260179}}{{cite journal |last1=Walsh |first1=James K. |last2=Perlis |first2=Michael |author-link2=Michael L. Perlis |last3=Rosenthal |first3=Murray |last4=Krystal |first4=Andrew |last5=Jiang |first5=John |last6=Roth |first6=Thomas |date=2006 |title=Tiagabine increases slow-wave sleep in a dose-dependent fashion without affecting traditional efficacy measures in adults with primary insomnia |url=https://pubmed.ncbi.nlm.nih.gov/17557435/ |journal=J Clin Sleep Med |volume=15 |issue=2(1) |pages=35–41|doi=10.5664/jcsm.26433 |pmid=17557435}}

Levodopa is a drug commonly used to treat Parkinson's disease which acts to increases the brain's dopamine availability. Nocturnal single doses of levodopa increase slow-wave sleep by 10.6% in the elderly.{{Cite journal |last1=Isotalus |first1=Hanna K. |last2=Carr |first2=Will J. |last3=Blackman |first3=Jonathan |last4=Averill |first4=George G. |last5=Radtke |first5=Oliver |last6=Selwood |first6=James |last7=Williams |first7=Rachel |last8=Ford |first8=Elizabeth |last9=McCullagh |first9=Liz |last10=McErlane |first10=James |last11=O'Donnell |first11=Cian |last12=Durant |first12=Claire |last13=Bartsch |first13=Ullrich |last14=Jones |first14=Matt W. |last15=Muñoz-Neira |first15=Carlos |date=2023 |title=L-DOPA increases slow-wave sleep duration and selectively modulates memory persistence in older adults |journal=Frontiers in Behavioral Neuroscience |volume=17 |pages=1096720 |doi=10.3389/fnbeh.2023.1096720 |issn=1662-5153 |pmid=37091594|pmc=10113484 |doi-access=free }}

Antagonists of certain serotonergic receptors (namely 5-HT_2A and 5-HT_2C) have also been demonstrated to enhance slow-wave sleep, although they do not consistently bring about improvements in overall sleep duration or symptoms associated with insomnia. Trazodone, an atypical antidepressant, increases the duration of low-wave sleep; it is suspected that trazodone's antagonistic action at the 5-HT_2A receptor may contribute to this effect.{{cite journal |last1=Suzuki |first1=H |last2=Yamadera |first2=H |last3=Nakamura |first3=S |last4=Endo |first4=S |title=Effects of trazodone and imipramine on the biological rhythm: an analysis of sleep EEG and body core temperature. |journal=Journal of Nippon Medical School |date=August 2002 |volume=69 |issue=4 |pages=333–41 |doi=10.1272/jnms.69.333 |pmid=12187365|doi-access=free }} A variety of drugs that antagonise the on 5-HT_2A and 5-HT_2C receptors exhibit SWS-enhancing effects in humans.{{cite journal |last1=Sharpley |first1=AL |last2=Elliott |first2=JM |last3=Attenburrow |first3=MJ |last4=Cowen |first4=PJ |title=Slow wave sleep in humans: role of 5-HT2A and 5-HT2C receptors. |journal=Neuropharmacology |date=March 1994 |volume=33 |issue=3–4 |pages=467–71 |doi=10.1016/0028-3908(94)90077-9 |pmid=7984285|s2cid=46587971 }}{{cite journal |last1=Dijk |first1=DJ |author-link1=Derk-Jan Dijk |title=Slow-wave sleep deficiency and enhancement: implications for insomnia and its management. |journal=The World Journal of Biological Psychiatry |date=June 2010 |volume=11 |issue=Suppl 1 |pages=22–8 |doi=10.3109/15622971003637645 |pmid=20509829|s2cid=37749231 }}