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Article: Sleep
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sleep is when you're tired.'''see asleep.'''''333'' no copyright. wikapedia
{{Redirect|Waking up||Waking Up (disambiguation)}}
 
{{Redirect|Asleep}}
 
{{Redirect|Slept||SLEPT analysis}}
 
 
{{Dablink|This article is about sleep in general, and is mostly concerned with sleep in humans; for specifically non-human sleep see [[Sleep (non-human)]]; for other uses, see [[Sleep (disambiguation)]].}}
 
[[File:A_child_sleeping.jpg|thumb|Sleeping child|alt=Image of a sleeping child's face]]
 
 
'''Sleep''' is a naturally recurring state characterized by reduced or absent [[consciousness]], relatively suspended sensory activity, and inactivity of nearly all [[voluntary muscle]]s.<ref>''Macmillan Dictionary for Students'' Macmillan, Pan Ltd. (1981), p. 936. Retrieved 2009-10-1.</ref> It is distinguished from quiet [[wakefulness]] by a decreased ability to react to [[stimulus (physiology)|stimuli]], and is more easily reversible than being in [[hibernation]] or a [[coma]]. Sleep is a heightened [[anabolic]] state, accentuating the growth and rejuvenation of the immune, nervous, skeletal and muscular systems. It is observed in all mammals, all birds, and many reptiles, amphibians, and fish.
 
 
The purposes and mechanisms of sleep are only partially clear and are the subject of intense research.<ref>{{cite web |url=http://thesciencenetwork.org/programs/waking-up-to-sleep |title=Waking Up To Sleep |accessdate=2008-01-25 |last=Bingham |first=Roger |coauthors=Terrence Sejnowski, Jerry Siegel, Mark Eric Dyken, Charles Czeisler, Paul Shaw, Ralph Greenspan, Satchin Panda, Philip Low, Robert Stickgold, Sara Mednick, Allan Pack, Luis de Lecea, David Dinges, Dan Kripke, Giulio Tononi |date=February 2007 |format=Several conference videos |publisher=The Science Network}}</ref> Sleep is often thought to help conserve energy,<ref name="syllabus">{{cite web |url=http://www.sleephomepages.org/sleepsyllabus/b.html |title=Sleep Syllabus. B. The Phylogeny of Sleep |accessdate=2010-09-26 |publisher=Sleep Research Society, Education Committee}}</ref><ref name="function">[http://www.scribd.com/doc/13916183/AQA-ALevel-Psychology-PYA4-Function-of-Sleep "Function of Sleep."]. Scribd.com. Retrieved on 2011-12-01.</ref> but decreases metabolism only about 5–10%.<ref name="syllabus"/><ref name="function"/> Hibernating animals need to sleep despite the hypometabolism seen in hibernation, and must return from [[hypothermia]] to [[euthermia]] in order to sleep, making sleeping "energetically expensive."<ref name="pmid1945046">{{cite journal |author=Daan S, Barnes BM, Strijkstra AM |title=Warming up for sleep? Ground squirrels sleep during arousals from hibernation |journal=Neurosci. Lett. |volume=128 |issue=2 |pages=265–8 |year=1991 |pmid=1945046 |doi=10.1016/0304-3940(91)90276-Y}}</ref>
 
{{TOC limit|limit=3}}
 
 
==Physiology==
 
===Sleep stages===
 
In mammals and birds, sleep is divided into two broad types: [[rapid eye movement sleep|rapid eye movement]] (REM) and [[non-rapid eye movement sleep|non-rapid eye movement]] (NREM or non-REM) sleep. Each type has a distinct set of associated physiological, neurological, and psychological features. The [[American Academy of Sleep Medicine]] (AASM) further divides NREM into three stages: N1, N2, and N3, the last of which is also called [[delta wave|delta]] sleep or [[slow-wave sleep]] (SWS).<ref>{{cite journal |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 |url=http://www.aasmnet.org/jcsm/Articles/030203.pdf |last1=Silber |first1=MH |last2=Ancoli-Israel |first2=S |last3=Bonnet |first3=MH |last4=Chokroverty |first4=S |last5=Grigg-Damberger |first5=MM |last6=Hirshkowitz |first6=M |last7=Kapen |first7=S |last8=Keenan |first8=SA |last9=Kryger |first9=MH}}</ref>
 
Humans experience four stages of sleep:
 
* Stage 1: [[Theta rhythm|theta activity]] is 3.5-7.5 Hz. This is a stage between sleep and wakefulness. The muscles are active, and the eyes roll slowly, opening and closing moderately.
 
* Stage 2: eyes dart back and forth.
 
* Stage 3: formerly divided into stages 3 and 4, is called [[slow wave sleep]] - which is caused by the preoptic area which consists of [[delta activity]], high amplitude waves at less than 3.5 Hz. The sleeper is less responsive to the environment; environmental stimuli that may have distracted them in stage one, no longer produce any reactions.
 
* The sleeper now enters rapid eye movement (REM) where most muscles are paralyzed. REM sleep is turned on by acetylcholine secretion and is inhibited by neurons that secrete serotonin. An adult reaches REM approximately every 90 minutes, with the latter half of sleep being more dominated by this stage. The function of REM sleep is uncertain but a lack of it will impair the ability to learn complex tasks. One approach to understand the role of sleep is to study the deprivation of it.<ref>psychology the science of behavior , Neil Carson, C. Donald Heth , 4th ed</ref>
 
 
[[File:Sleep Hypnogram.svg|thumb|319px|Hypnogram showing sleep cycles from midnight to 6.30 am, with deep sleep early on. There is more REM (marked red) before waking.|alt=Hypnogram showing sleep cycles from midnight to morning]]
 
[[File:Sleep EEG Stage 4.jpg|thumb|right|Stage N3 sleep; [[EEG]] highlighted by red box. Thirty seconds of deep sleep, here with greater than 50% delta waves.]]
 
[[File:Sleep EEG REM.png|thumb|right|[[Rapid eye movement (sleep)|REM sleep]]; EEG highlighted by red box; eye movements highlighted by red line. Thirty seconds of sleep.]]
 
Sleep proceeds in cycles of REM and NREM, usually four or five of them per night, the order normally being N1 → N2 → N3 → N2 → REM. 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.
 
 
The stages of sleep were first described in 1937 by [[Alfred Lee Loomis]] and his coworkers, who separated the different [[electroencephalography]] (EEG) features of sleep into five levels (A to E), which represented the spectrum from wakefulness to deep sleep.<ref>{{cite journal |last=Loomis |first=Alfred L |coauthors=Harvey EN, Hobart GA |title=III Cerebral states during sleep, as studied by human brain potentials |journal=J Exp Psychol. |year=1937 |volume=21 |pages=127–44 |doi=10.1037/h0057431 |issue=2}}</ref> In 1953, REM sleep was discovered as distinct, and thus [[William Dement]] and [[Nathaniel Kleitman]] reclassified sleep into four NREM stages and REM.<ref>{{cite journal |last=Dement |first=William |coauthor=Nathaniel Kleitman |title=Cyclic variations in EEG during sleep and their relation to eye movements, body motility and dreaming |journal=Electroencephalogr Clin Neurophysiol |year=1957 |volume=9 |pages=673–90 |doi=10.1016/0013-4694(57)90088-3 |pmid=13480240 |issue=4}}</ref> The staging criteria were standardized in 1968 by [[Allan Rechtschaffen]] and [[Anthony Kales]] in the "R&K sleep scoring manual."<ref>{{cite book | url=http://commons.wikimedia.org/wiki/File:Rechschaffen-Kales---Manual.pdf | author=Rechtschaffen A, Kales A, editors. | title=A Manual of Standardized Terminology, Techniques and Scoring System for Sleep Stages of Human Subjects. | publisher=Washington: Public Health Service, US Government Printing Office | year=1968 | format = PDF}}</ref> In the R&K standard, NREM sleep was divided into four stages, with slow-wave sleep comprising stages 3 and 4. In stage 3, delta waves made up less than 50% of the total wave patterns, while they made up more than 50% in stage 4. Furthermore, REM sleep was sometimes referred to as stage 5.
 
 
In 2004, the AASM commissioned the AASM Visual Scoring Task Force to review the R&K scoring system. The review resulted in several changes, the most significant being the combination of stages 3 and 4 into Stage N3. The revised scoring was published in 2007 as ''The AASM Manual for the Scoring of Sleep and Associated Events''.<ref>{{cite book |last=Iber |first=C |coauthors=Ancoli-Israel, S; Chesson, A; Quan, SF for the American Academy of Sleep Medicine |title=The AASM Manual for the Scoring of Sleep and Associated Events: Rules, Terminology and Technical Specifications |location=Westchester |publisher=American Academy of Sleep Medicine |year=2007}}</ref> Arousals and respiratory, cardiac, and movement events were also added.<ref>{{cite web |url=http://web.mst.edu/~psyworld/general/sleepstages/sleepstages.pdf |title= Stages of Sleep |accessdate=2008-06-15 |author=Psychology World |year=1998 |format=PDF |quote=(includes illustrations of "sleep spindles" and "K-complexes")}}</ref><ref>{{cite journal |author=Schulz H |title= Rethinking sleep analysis |journal=Journal of Clinical Sleep Medicine |volume=4 |issue=2 |pages=99–103 |date=April 2008 |pmid=18468306 |pmc=2335403}}</ref>
 
 
Sleep stages and other characteristics of sleep are commonly assessed by [[polysomnography]] in a specialized sleep laboratory. Measurements taken include EEG of [[neural oscillations|brain waves]], [[electrooculography]] (EOG) of eye movements, and [[electromyography]] (EMG) of [[skeletal muscle]] activity. In humans, the average length of the first sleep cycle is approximately 90 minutes and 100 to 120 minutes from the second to the fourth cycle, which is usually the last one.<ref>{{cite book|publisher=Springer|author=C. Guilleminault|coauthors=M.L. Kreutzer|editor=Michael Billiard|accessdate=2012-04-07|url=http://books.google.com/books?id=IorPrIY6dOYC&lpg=PA|page=5|language=English|chapter=Chapter 1 – Normal Sleep|quote=The average length of the first sleep cycle is approximately 90 minutes and 100 to 120 minutes from the second to the fourth cycle, which is usually the last one.|title=Sleep: Physiology, Investigations, and Medicine (Google eBook)|isbn=978-0-306-47406-4|date=2003-09-30}}</ref> Each stage may have a distinct physiological function and this can result in sleep that exhibits [[unconsciousness|loss of consciousness]] but does not fulfill its physiological functions (i.e., one may still feel tired after apparently sufficient sleep).
 
 
Scientific studies on sleep have shown that sleep stage at awakening is an important factor in amplifying [[sleep inertia]]. [[Alarm clock]]s involving ''sleep stage monitoring'' appeared on the market in 2005.<ref>{{cite news |url=http://www.reuters.com/article/technologyNews/idUSL0878172320070829 |title=Bio-alarm clocks set for perfect wake-up |accessdate=2008-06-09 |author=Reuven Fenton |work= Reuters |date=2007-08-29}}</ref> Using sensing technologies such as [[EEG]] electrodes or [[accelerometers]], these alarm clocks are supposed to wake people only from light sleep.
 
 
===NREM sleep===
 
{{Main|Non-rapid eye movement sleep}}
 
According to the 2007 AASM standards, NREM consists of three stages. There is relatively little dreaming in NREM.
 
 
'''Stage N1''' refers to the transition of the brain from [[alpha waves]] having a frequency of 8–13&nbsp;[[Hertz|Hz]] (common in the awake state) to [[theta wave]]s having a frequency of 4–7&nbsp;Hz. This stage is sometimes referred to as somnolence or drowsy sleep. Sudden twitches and [[hypnic jerk]]s, also known as positive [[myoclonus]], may be associated with the onset of sleep during N1. Some people may also experience [[hypnagogic hallucination]]s during this stage. During N1, the subject loses some [[muscle tone]] and most conscious awareness of the external environment.
 
 
'''Stage N2''' is characterized by [[sleep spindle]]s ranging from 11 to 16&nbsp;Hz (most commonly 12–14&nbsp;Hz) and [[K-complex]]es. During this stage, muscular activity as measured by EMG decreases, and conscious awareness of the external environment disappears. This stage occupies 45–55% of total sleep in adults.
 
 
'''Stage N3''' (deep or [[slow-wave sleep]]) is characterized by the presence of a minimum of 20% [[delta wave]]s ranging from 0.5–2&nbsp;Hz and having a peak-to-peak amplitude >75 μV. (EEG standards define delta waves to be from 0 to 4&nbsp;Hz, but sleep standards in both the original R&K, as well as the new 2007 AASM guidelines have a range of 0.5–2&nbsp;Hz.) This is the stage in which [[parasomnia]]s such as [[night terror]]s, [[nocturnal enuresis]], [[sleepwalking]], and [[somniloquy]] occur. Many illustrations and descriptions still show a stage N3 with 20–50% delta waves and a stage N4 with greater than 50% delta waves; these have been combined as stage N3.
 
 
===REM sleep===
 
{{Main|Rapid eye movement sleep}}
 
Rapid eye movement sleep, or REM sleep (also known as paradoxical sleep),<ref name="Myers2003">{{cite book|author=David G. Myers|title=Psychology, Seventh Edition, in Modules (High School Version)|url=http://books.google.com/books?id=oYuBwPDsQZoC&pg=PA268|accessdate=22 August 2012|date=22 September 2003|publisher=Macmillan|isbn=978-0-7167-8595-8|pages=268–}}</ref> accounts for 20–25% of total sleep time in most human adults. The criteria for REM sleep include rapid eye movements as well as a rapid low-voltage EEG. Most memorable dreaming occurs in this stage. At least in mammals, a descending muscular [[atonia]] is seen. Such paralysis may be necessary to protect organisms from self-damage through physically acting out scenes from the often-vivid dreams that occur during this stage.{{Citation needed|date=June 2012}}
 
 
===Timing===
 
[[File:Biological clock human.PNG|thumb|450px|The human biological clock]]
 
Sleep timing is controlled by the [[circadian clock]], sleep-wake [[homeostasis]], and in humans, within certain bounds, willed behavior. The circadian clock—an inner timekeeping, temperature-fluctuating, enzyme-controlling device—works in tandem with [[adenosine]], a neurotransmitter that inhibits many of the bodily processes associated with wakefulness. Adenosine is created over the course of the day; high levels of adenosine lead to sleepiness.<ref>http://thebrain.mcgill.ca/flash/a/a_11/a_11_m/a_11_m_cyc/a_11_m_cyc.html</ref> In [[diurnality|diurnal]] animals, sleepiness occurs as the circadian element causes the release of the hormone [[melatonin]] and a gradual decrease in core [[thermoregulation|body temperature]]. The timing is affected by one's [[chronotype]]. It is the circadian rhythm that determines the ideal timing of a correctly structured and restorative sleep episode.<ref>{{cite journal |last=Wyatt |first=James K. |coauthors=Ritz-De Cecco, Angela; Czeisler, Charles A.; Dijk, Derk-Jan |date=1 October 1999 |title=Circadian temperature and melatonin rhythms, sleep, and neurobehavioral function in humans living on a 20-h day |journal=Am J Physiol |volume=277 |issue=4 |pages=R1152–R1163|pmid=10516257}}</ref>
 
 
Homeostatic sleep propensity (the need for sleep as a function of the amount of time elapsed since the last adequate sleep episode) must be balanced against the circadian element for satisfactory sleep.<ref name="Zisapel">{{cite journal |last=Zisapel |first=N |year=2007 |title=Sleep and sleep disturbances: biological basis and clinical implications |journal=Cell Mol Life Sci |volume=64 |issue=10 |pages=1174–86 |pmid=17364142 |doi=10.1007/s00018-007-6529-9}}</ref> Along with corresponding messages from the circadian clock, this tells the body it needs to sleep.<ref name="autogenerated1">{{cite web |url=http://www.helpguide.org/life/sleeping.htm |title=Understanding Sleep: Sleep Needs, Cycles, and Stages |accessdate=2008-01-25 |last=de Benedictis |first=Tina, PhD |coauthors=Heather Larson, Gina Kemp, MA, Suzanne Barston, Robert Segal, MA |year=2007 |publisher=Helpguide.org}}</ref> Sleep offset (awakening) is primarily determined by circadian rhythm. A person who regularly awakens at an early hour will generally not be able to sleep much later than his or her normal waking time, even if moderately sleep-deprived.
 
 
Sleep duration is affected by the gene [[DEC2]]. Some people have a mutation of this gene; they sleep two hours less than normal. Neurology professor Ying-Hui Fu and her colleagues bred mice that carried the DEC2 mutation and slept less than normal mice.<ref>{{cite web |url=http://www.medicinenet.com/script/main/art.asp?articlekey=104720|archiveurl=http://web.archive.org/web/20110714065601/http://www.medicinenet.com/script/main/art.asp?articlekey=104720|archivedate=2011-07-14|title=Gene Cuts Need for Sleep - Sleep Disorders Including, Sleep Apnea, Narcolepsy, Insomnia, Snoring and Nightmares on MedicineNet.com |accessdate=2010-06-11}}</ref><ref>{{cite journal |title=The transcriptional repressor DEC2 regulates sleep length in mammals |journal=Science |volume=325 |issue=5942 |pages=866–70 |year=2009 |pmid=19679812 |pmc=2884988 |doi=10.1126/science.1174443 |last1=He |first1=Y. |last2=Jones |first2=C. R. |last3=Fujiki |first3=N. |last4=Xu |first4=Y. |last5=Guo |first5=B. |last6=Holder |first6=J. L. |last7=Rossner |first7=M. J. |last8=Nishino |first8=S. |last9=Fu |first9=Y.-H.}}</ref>
 
 
===Optimal amount in humans===
 
====Adult====
 
The optimal amount of sleep is not a meaningful concept unless the timing of that sleep is seen in relation to an individual's [[circadian rhythm]]s. A person's major sleep episode is relatively inefficient and inadequate when it occurs at the "wrong" time of day; one should be asleep at least six hours before the lowest body temperature.<ref>{{cite journal |last=Dijk |first=Derk-Jan |coauthor=Steven W. Lockley |date= February 2002 |title=Functional Genomics of Sleep and Circadian Rhythm Invited Review: Integration of human sleep-wake regulation and circadian rhythmicity |journal=J Appl Physiol |volume=92 |issue=2 |pages= 852–62 |pmid=11796701|quote=Consolidation of sleep for 8 h or more is only observed when sleep is initiated ~6–8 h before the temperature nadir. |doi=10.1152/japplphysiol.00924.2001 }}</ref> The timing is correct when the following two circadian markers occur after the middle of the sleep episode and before awakening:<ref>{{cite journal |last=Wyatt |first=James K. |coauthors=Ritz-De Cecco, Angela; Czeisler, Charles A.; Dijk, Derk-Jan |date=1 October 1999 |title=Circadian temperature and melatonin rhythms, sleep, and neurobehavioral function in humans living on a 20-h day |journal=Am J Physiol |volume=277 |issue=4 |pages=R1152–R1163 |quote=... significant homeostatic and circadian modulation of sleep structure, with the highest sleep efficiency occurring in sleep episodes bracketing the melatonin maximum and core body temperature minimum |pmid=10516257}}</ref>
 
 
* maximum concentration of the hormone melatonin
 
* minimum core body temperature.
 
 
For more information on the human circadian rhythm and body temperature, see [[Circadian rhythm#Biological markers|Biological markers]] (in the article [[Circadian rhythm]]).
 
 
Human sleep needs can vary by age and among individuals, and sleep is considered to be adequate when there is no daytime sleepiness or dysfunction. Moreover, self-reported sleep duration is only moderately correlated with actual sleep time as measured by [[actigraphy]],<ref>{{cite journal|author=Lauderdale, Diane S.; Knutson, Kristen L.; Yan, Lijing L.; Liu, Kiang; Rathouz, Paul J. |title=Self-Reported and Measured Sleep Duration: How Similar Are They?|doi= 10.1097/EDE.0b013e318187a7b0|year=2008|journal=Epidemiology|volume=19|issue=6|pages=838–45|pmid=18854708|pmc=2785092}}</ref> and those affected with [[sleep state misperception]] may typically report having slept only four hours despite having slept a full eight hours.<ref name=healthcom>[http://www.sleepdisorderchannel.com/insomnia/causes.shtml Insomnia Causes]. Healthcommunities.com. Original Publication: 01 Dec 2000. Updated: 01 Dec 2007.</ref>
 
 
A [[University of California, San Diego]] psychiatry study of more than one million adults found that people who live the longest self-report sleeping for six to seven hours each night.<ref>{{cite news |author= Rhonda Rowland |title=Experts challenge study linking sleep, life span |date=2002-02-15 |url=http://archives.cnn.com/2002/HEALTH/02/14/sleep.study/index.html |accessdate=2007-04-22 |work=CNN}}</ref> Another study of sleep duration and mortality risk in women showed similar results.<ref>{{cite journal |title=A prospective study of sleep duration and mortality risk in women |journal=Sleep |volume=27 |issue=3 |pages=440–4 |date=May 2004 |pmid=15164896 |last1=Patel |first1=SR |last2=Ayas |first2=NT |last3=Malhotra |first3=MR |last4=White |first4=DP |last5=Schernhammer |first5=ES |last6=Speizer |first6=FE |last7=Stampfer |first7=MJ |last8=Hu |first8=FB }}</ref> Other studies show that "sleeping more than 7 to 8 hours per day has been consistently associated with increased mortality," though this study suggests the cause is probably other factors such as depression and socioeconomic status, which would correlate statistically.<ref>{{cite journal |author=Patel SR, Malhotra A, Gottlieb DJ, White DP, Hu FB |title=Correlates of long sleep duration |journal=Sleep |volume=29 |issue=7 |pages=881–9 |date=July 2006 |pmid=16895254}}; cf. {{cite journal |author=Irwin MR, Ziegler M |title=Sleep deprivation potentiates activation of cardiovascular and catecholamine responses in abstinent alcoholics |journal=Hypertension |volume=45 |issue=2 |pages=252–7 |date=February 2005 |pmid=15642774 |doi=10.1161/01.HYP.0000153517.44295.07}}</ref> It has been suggested that the correlation between lower sleep hours and reduced morbidity only occurs with those who wake naturally, rather than those who use an alarm.
 
 
[[File:Effects of sleep deprivation.svg|thumb|300px|Main health effects of [[sleep deprivation]],<ref>Reference list is found on image page in Commons: [[:Commons:File:Effects of sleep deprivation.svg#References]]</ref> indicating impairment of normal maintenance by sleep]]
 
Researchers at the [[University of Warwick]] and [[University College London]] have found that lack of sleep can more than double the risk of death from [[cardiovascular disease]], but that too much sleep can also be associated with a doubling of the risk of death, though not primarily from cardiovascular disease.<ref>{{cite web |title=Researchers say lack of sleep doubles risk of death... but so can too much sleep |url=http://www2.warwick.ac.uk/newsandevents/pressreleases/researchers_say_lack/}}</ref><ref>{{cite journal|title=A prospective study of change in sleep duration: associations with mortality in the Whitehall II cohort |journal=Sleep |volume=30 |issue=12 |pages=1659–66 |date=December 2007 |pmid=18246975 |pmc=2276139|last1=Ferrie|first1=JE|last2=Shipley|first2=MJ|last3=Cappuccio|first3=FP|last4=Brunner|first4=E|last5=Miller|first5=MA|last6=Kumari|first6=M|last7=Marmot|first7=MG }}</ref> Professor Francesco Cappuccio said, "Short sleep has been shown to be a [[risk factor]] for weight gain, [[hypertension]], and [[diabetes mellitus type 2|Type 2 diabetes]], sometimes leading to mortality; but in contrast to the short sleep-mortality association, it appears that no potential mechanisms by which long sleep could be associated with increased mortality have yet been investigated. Some candidate causes for this include depression, low socioeconomic status, and [[cancer-related fatigue]]... In terms of prevention, our findings indicate that consistently sleeping around seven hours per night is optimal for health, and a sustained reduction may predispose to ill health."
 
 
Furthermore, sleep difficulties are closely associated with psychiatric disorders such as [[major depressive disorder|depression]], [[alcoholism]], and [[bipolar disorder]].<ref name="Thase2006">{{cite pmid|16889107}}</ref> Up to 90% of adults with depression are found to have sleep difficulties. Dysregulation found on EEG includes disturbances in sleep continuity, decreased delta sleep and altered REM patterns with regard to latency, distribution across the night and density of eye movements.<ref>{{cite book |last=Mann |first=Joseph John |coauthor=David J. Kupfer |title=Biology of Depressive Disorders: Subtypes of depression and comorbid disorders, Part 2 |url=http://books.google.com/?id=qbbTmje6oskC&printsec=frontcover |format=Google books |accessdate=2009-07-24 |year=1993 |publisher=Springer |page=49 |isbn= 0-306-44296-5}}</ref>
 
 
====Hours by age====
 
Children need more sleep per day in order to develop and function properly: up to 18 hours for [[newborn]] babies, with a declining rate as a child ages.<ref name="autogenerated1"/> A newborn baby spends almost 9 hours a day in REM sleep. By the age of five or so, only slightly over two hours is spent in REM. Studies say that school age children need about 10 to 11 hours of sleep.<ref>{{cite web |last=Siegel |first=Jerome M |url=http://www.npi.ucla.edu/sleepresearch/encarta/Article.htm |title=Sleep |accessdate=2008-01-25 |year=1999 |work=Encarta Encyclopedia |publisher=Microsoft |archiveurl=http://web.archive.org/web/20071214003754/http://www.npi.ucla.edu/sleepresearch/encarta/Article.htm <!--Bot retrieved archive--> |archivedate=2007-12-14}}</ref>
 
 
{| class="wikitable"
 
|-
 
! Age and condition
 
! Sleep Needs
 
|-
 
| Newborns (0–2 months)
 
| 12 to 18 hours<ref name="sleepfoundation">{{cite web |url=http://www.sleepfoundation.org/article/how-sleep-works/how-much-sleep-do-we-really-need |title=How Much Sleep Do We Really Need? |accessdate=2012-04-16 |date=Undated |publisher=[[National Sleep Foundation]]}}</ref>
 
|-
 
| Infants (3–11 months)
 
| 14 to 15 hours<ref name="sleepfoundation" />
 
|-
 
| Toddlers (1–3 years)
 
| 12 to 14 hours<ref name="sleepfoundation" />
 
|-
 
| Preschoolers (3–5 years)
 
| 11 to 13 hours<ref name="sleepfoundation" />
 
|-
 
| School-age children (5–10 years)
 
| 10 to 11 hours<ref name="sleepfoundation" />
 
|-
 
| [[Adolescent]]s (10–17 years)
 
| 8.5 to 9.25 hours<ref name="sleepfoundation" /><ref>{{cite web |url=http://www.sleepfoundation.org/article/hot-topics/backgrounder-later-school-start-times |title=Backgrounder: Later School Start Times |accessdate=2009-10-02 |date=Undated |publisher=[[National Sleep Foundation]] |quote=Teens are among those least likely to get enough sleep; while they need on average 9{{frac|1|4}} hours of sleep per night...}}</ref>
 
|-
 
| Adults, including elderly
 
| 7 to 9 hours<ref name="sleepfoundation" />
 
|-
 
| Pregnant women
 
| 8(+) hours
 
|}
 
 
==Siesta or nap==
 
The [[siesta]] habit has recently been associated with a 37% reduction in coronary mortality, possibly due to reduced cardiovascular stress mediated by daytime sleep.<ref name=Naska/> Nevertheless, epidemiological studies on the relations between cardiovascular health and siesta have led to conflicting conclusions, possibly because of poor control of moderator variables, such as physical activity. It is possible that people who take a siesta have different physical activity habits, e.g., waking earlier and scheduling more activity during the morning. Such differences in physical activity may mediate different 24-hour profiles in cardiovascular function. Even if such effects of physical activity can be discounted for explaining the relationship between siesta and cardiovascular health, it is still unknown whether it is the daytime nap itself, a supine posture or the expectancy of a nap that is the most important factor. It was recently suggested that a short nap can reduce stress and blood pressure (BP), with the main changes in BP occurring between the time of lights off and the onset of stage 1.<ref name=Zaregarizi1/><ref name=Zaregarizi2/>
 
Dr. Zaregarizi and his team have concluded that the acute time of falling asleep was where beneficial cardiovascular changes take place. This study has indicated that a large decline in blood pressure occurs during the daytime sleep-onset period only when sleep is expected however when subjects rest in a supine position, the same reduction in blood pressure is not observed. This blood pressure reduction may be associated with the lower coronary mortality rates seen in Mediterranean and Latin American populations where siestas are common.
 
Dr. Zaregarizi assessed cardiovascular function (blood pressure, heart rate, and measurements of blood vessel dilation) while nine healthy volunteers, 34 years of age on average, spent an hour standing quietly; reclining at rest but not sleeping; or reclining to nap. All participants were restricted to 4 hours of sleep on the night prior to each of the sleep laboratory tests. During the three phases of daytime sleep, he noted significant reductions in blood pressure and heart rate. By contrast, they did not observe changes in cardiovascular function while the participants were standing or reclining at rest.
 
These findings also show that the greatest decline in blood pressure occurs between lights-off and onset of daytime sleep itself.
 
During this sleep period, which lasted 9.7 minutes on average, blood pressure decreased, while blood vessel dilation increased by more than 9 percent.
 
“There is little change in blood pressure once a subject is actually asleep," Dr. Zaregarizi noted, and he found minor changes in blood vessel dilation during sleep.<ref name=Zaregarizi1/><ref name=Zaregarizi2/>
 
 
==Sleep debt==
 
{{Main|Sleep debt}}
 
Sleep debt is the effect of not getting enough sleep; a large debt causes mental, emotional and physical fatigue.
 
 
Sleep debt results in diminished abilities to perform high-level cognitive functions. Neurophysiological and functional [[imaging studies]] have demonstrated that frontal regions of the brain are particularly responsive to homeostatic sleep pressure.<ref>{{cite journal |author=Gottselig JM, Adam M, Rétey JV, Khatami R, Achermann P, Landolt HP |title=Random number generation during sleep deprivation: effects of caffeine on response maintenance and stereotypy |journal=Journal of Sleep Research |volume=15 |issue=1 |pages=31–40 |date=March 2006 |pmid=16490000 |doi=10.1111/j.1365-2869.2006.00497.x}}</ref>
 
 
Scientists do not agree on how much sleep debt it is possible to accumulate; whether it is accumulated against an individual's average sleep or some other benchmark; nor on whether the prevalence of sleep debt among adults has changed appreciably in the [[developed country|industrialized world]] in recent decades. It is likely that children are sleeping less than previously in [[Western world|Western societies]].<ref>{{cite journal |author=Iglowstein I, Jenni OG, Molinari L, Largo RH |title=Sleep duration from infancy to adolescence: reference values and generational trends |journal=Pediatrics |volume=111 |issue=2 |pages=302–7 |date=February 2003 |pmid=12563055 |quote=Thus, the shift in the evening bedtime across cohorts accounted for the substantial decrease in sleep duration in younger children between the 1970s and the 1990s... [A] more liberal parental attitude toward evening bedtime in the past decades is most likely responsible for the bedtime shift and for the decline of sleep duration... |doi=10.1542/peds.111.2.302}}</ref>
 
 
==Genetics==
 
It is suspected that a considerable amount of sleep-related behavior, such as when and how long a person needs to sleep, is regulated by our genetics. Researchers have discovered some evidence that seems to support this assumption.<ref>{{cite journal|doi=10.1126/science.1174443 |title=The Transcriptional Repressor DEC2 Regulates Sleep Length in Mammals|year=2009|last1=He|first1=Y.|last2=Jones|first2=C. R.|last3=Fujiki|first3=N.|last4=Xu|first4=Y.|last5=Guo|first5=B.|last6=Holder|first6=J. L.|last7=Rossner|first7=M. J.|last8=Nishino|first8=S.|last9=Fu|first9=Y.-H.|journal=Science|volume=325|issue=5942|pages=866–70|pmid=19679812|pmc=2884988}}</ref> [[ABCC9]] is one gene found which influences the duration of human sleep.<ref name="The ABCC9 of Sleep: A Genetic Factor Regulates How Long We Sleep">{{cite web|title=The ABCC9 of Sleep: A Genetic Factor Regulates How Long We Sleep|url=http://www.sciencedaily.com/releases/2011/11/111124150237.htm|publisher=Science Daily|accessdate=21 August 2012}}</ref>
 
 
==Functions==
 
[[Image:Sleeping Kutchi.jpg|thumb|right|A [[Kutchi people|Kutchi]] woman sleeping]]
 
{{See|Neuroscience of sleep#Sleep function}}
 
The multiple theories proposed to explain the function of sleep reflect the as-yet incomplete understanding of the subject. (When asked, after 50 years of research, what he knew about the reason people sleep, [[William Dement]], founder of [[Stanford University]]'s Sleep Research Center, answered, "As far as I know, the only reason we need to sleep that is really, really solid is because we get sleepy.")<ref name="NatGeoMay2010">Max, D. T. [http://ngm.nationalgeographic.com/2010/05/sleep/max-text The Secrets of Sleep] ''National Geographic Magazine'', May 2010.</ref> It is likely that sleep evolved to fulfill some primeval function and took on multiple functions over time{{Citation needed|date=April 2012}} (analogous to the [[larynx]], which controls the passage of food and air, but [[Larynx#Descended larynx|descended]] over time to develop speech capabilities).
 
 
If sleep were not essential, one would expect to find:
 
 
* Animal species that do not sleep at all
 
* Animals that do not need recovery sleep when they stay awake longer than usual
 
* Animals that suffer no serious consequences as a result of lack of sleep
 
 
Outside of a few [[Basal (phylogenetics)|basal]] animals that have no brain or a very simple one, no animals have been found to date that satisfy any of these criteria.<ref>{{cite journal |last=Cirelli |first=Chiara |coauthor=Giulio Tononi |date=August 26, 2008 |title=Is Sleep Essential? |journal=PLoS Biol |volume=6 |issue=8 |page=e216 |publisher=Public Library of Science |pmid=18752355 |doi=10.1371/journal.pbio.0060216 |format=Essay |quote=... it would seem that searching for a core function of sleep, particularly at the cellular level, remains a worthwhile exercise |pmc=2525690}}</ref> While some varieties of shark, such as great whites and hammerheads, must remain in motion at all times to move oxygenated water over their gills, it is possible they still sleep one cerebral hemisphere at a time as marine mammals do. However it remains to be shown definitively whether any fish is capable of [[Unihemispheric slow-wave sleep|unihemispheric sleep]].
 
 
Some of the many proposed functions of sleep are as follows:
 
 
===Restoration===
 
[[Wound healing]] has been shown to be affected by sleep. A study conducted by Gumustekin et al.<ref>{{cite journal |title=Effects of sleep deprivation, nicotine, and selenium on wound healing in rats |author=Gumustekin, K.; Seven, B.; Karabulut, N.; Aktas, O.; Gursan, N.; Aslan, S.; Keles, M.; Varoglu, E.; Dane, S. |year=2004 |journal=Int J Neurosci|volume=114|issue=11|pages=1433–42 |doi=10.1080/00207450490509168 |pmid=15636354}}</ref> in 2004 shows [[sleep deprivation]] hindering the [[healing]] of burns on rats.
 
 
It has been shown that sleep deprivation affects the [[immune system]]. In a study by Zager et al. in 2007,<ref>{{cite journal |author=Zager, A.; Andersen, M.L.; Ruiz, F.S.; Antunes, I.B. and Tufik, S. |year=2007|title= Effects of acute and chronic sleep loss on immune modulation of rats |journal=Regulatory, Integrative and Comparative Physiology|volume= 293|pages=R504–R509 |doi=10.1152/ajpregu.00105.2007}}</ref> rats were deprived of sleep for 24 hours. When compared with a [[experimental control|control group]], the sleep-deprived rats' [[blood test]]s indicated a 20% decrease in [[white blood cell]] count, a significant change in the immune system. It is now possible to state that "sleep loss impairs immune function and immune challenge alters sleep," and it has been suggested that mammalian species which invest in longer sleep times are investing in the immune system, as species with the longer sleep times have higher white blood cell counts.<ref>{{cite journal |last=Opp |first=Mark R |date=January 2009 |title=Sleeping to fuel the immune system: mammalian sleep and resistance to parasites |journal=BMC Evolutionary Biology |volume=9 |pmid=19134176 |pages=1471–2148 |pmc=2633283 |publisher=BioMed Central Ltd. |doi=10.1186/1471-2148-9-8 |url=http://www.biomedcentral.com/1471-2148/9/8 |format=Full text, Creative Commons Attribution License |accessdate=2009-06-28}}</ref> Sleep has also been theorized to effectively combat the accumulation of free radicals in the brain, by increasing the efficiency of endogeneous antioxidant mechanisms.<ref>{{cite journal|last=Reimund|first=E.|journal=Medical Hypotheses|year=1994|month=October|volume=45|issue=4|pages=231–3|pmid=7838006|doi=10.1016/0306-9877(94)90071-X|title=The free radical flux theory of sleep}}</ref>
 
 
It has yet to be proven that sleep duration affects [[somatic]] growth. One study by Jenni et al.<ref>{{cite journal |author=Jenni, O.G.; Molinari, L.; Caflisch, J.A.; Largo, R.H. |year=2007|title= Sleep duration from ages 1 to 10 years: Variability and stability in comparison with growth |journal=Pediatrics|volume= 120|pages= e769–e776 |doi=10.1542/peds.2006-3300 |pmid=17908734 |issue=4}}</ref> in 2007 recorded growth, height, and weight, as correlated to parent-reported time in bed in 305 children over a period of nine years (age 1–10). It was found that "the variation of sleep duration among children does not seem to have an effect on growth." It has been shown that sleep—more specifically, slow-wave sleep (SWS)—does affect [[growth hormone]] levels in adult men. During eight hours' sleep, Van Cauter, Leproult, and Plat<ref>{{cite journal |author=Van Cauter, E.; Leproult, R.; Plat, L. |year=2000|title=Age-related changes in slow-wave sleep and REM sleep and relationship with growth hormone and cortisol levels in healthy men |journal=Journal of the American Medical Association|volume=284|pages= 861–868 |doi=10.1001/jama.284.7.861 |pmid=10938176 |issue=7}}</ref> found that the men with a high percentage of SWS (average 24%) also had high growth hormone secretion, while subjects with a low percentage of SWS (average 9%) had low growth hormone secretion.
 
 
There are multiple arguments supporting the restorative function of sleep. The metabolic phase during sleep is anabolic; anabolic hormones such as growth hormones (as mentioned above) are secreted preferentially during sleep. The duration of sleep among species is, in general, [[inverse relationship|inversely related]] to animal size{{Citation needed|date=September 2011}} and directly related to [[basal metabolic rate]]. Rats with a very high basal metabolic rate sleep for up to 14 hours a day, whereas elephants and giraffes with lower BMRs sleep only 3–4 hours per day.
 
 
Energy conservation could as well have been accomplished by resting quiescent without shutting off the organism from the environment, potentially a dangerous situation. A sedentary nonsleeping animal is more likely to survive predators, while still preserving energy. Sleep, therefore, seems to serve another purpose, or other purposes, than simply conserving energy; for example, [[hibernating]] animals waking up from hibernation go into rebound sleep because of lack of sleep during the hibernation period. They are definitely well-rested and are conserving energy during hibernation, but need sleep for something else.<ref name="pmid1945046">{{cite journal |author=Daan S, Barnes BM, Strijkstra AM |title=Warming up for sleep? Ground squirrels sleep during arousals from hibernation |journal=Neurosci. Lett. |volume=128 |issue=2 |page=581 |year=1991 |pmid=1945046 |doi=10.1016/0304-3940(91)90276-Y |pages=265–8}}</ref> Rats kept awake indefinitely develop skin lesions, [[hyperphagia]], loss of body mass, [[hypothermia]], and, eventually, fatal [[sepsis]].<ref>{{cite book |others=Institute for Laboratory Animal Research (ILAR), National Research Council |title=Guidelines for the Care and Use of Mammals in Neuroscience and Behavioral Research |url=http://books.nap.edu/openbook.php?record_id=10732&page=121 |year=2003 |publisher=The National Academies Press |isbn=978-0-309-08903-6 |page=121 |quote=Sleep deprivation of over 7 days with the [[disk-over-water system]] results in the development of ulcerative skin lesions, hyperphagia, loss of body mass, hypothermia, and eventually septicemia and death in rats (Everson, 1995; Rechtschaffen et al., 1983).}}</ref>
 
 
[[File:A_baby_sleeping.jpg|thumb|left|upright|Infants spend most of their time sleeping, and most of that sleep is REM sleep.]]
 
===Ontogenesis===
 
According to the [[ontogenetic]] hypothesis of REM sleep, the activity occurring during [[neonatal]] REM sleep (or active sleep) seems to be particularly important to the developing organism (Marks et al., 1995). Studies investigating the effects of deprivation of active sleep have shown that deprivation early in life can result in behavioral problems, permanent sleep disruption, decreased brain mass (Mirmiran et al., 1983), and an abnormal amount of neuronal cell death.<ref name=Morrissey/>
 
 
REM sleep appears to be important for development of the brain. REM sleep occupies the majority of time of sleep of infants, who spend most of their time sleeping. Among different species, the more immature the baby is born, the more time it spends in REM sleep. Proponents also suggest that REM-induced muscle inhibition in the presence of brain activation exists to allow for brain development by activating the synapses, yet without any motor consequences that may get the infant in trouble. Additionally, REM deprivation results in developmental abnormalities later in life.
 
 
However, this does not explain why older adults still need REM sleep. [[Marine mammal|Aquatic mammal]] infants do not have REM sleep in infancy;<ref>{{cite web |title=Why do we Sleep? |url=http://www.slate.com/id/2162475/entry/2162477/ |publisher=Slate.com |date=May 27, 2007 |accessdate=2008-08-23 |author=Amanda Schaffer}}</ref> REM sleep in those animals increases as they age.
 
 
===Memory processing===
 
{{See|Sleep and learning|Sleep and creativity|Sleep and memory}}
 
Scientists have shown numerous ways in which sleep is related to [[memory]]. In a study conducted by Turner, Drummond, Salamat, and Brown,<ref>{{cite journal|author=Turner, T.H.; Drummond, S.P.A.; Salamat, J.S.; Brown, G.G. |year=2007|title=Effects of 42 hr sleep deprivation on component processes of verbal working memory|journal=Neuropsychology|volume=21|pages=787–795|doi=10.1037/0894-4105.21.6.787|pmid=17983292|issue=6}}</ref> [[working memory]] was shown to be affected by sleep deprivation. Working memory is important because it keeps information active for further processing and supports higher-level [[cognitive functions]] such as [[decision making]], [[reasoning]], and [[episodic memory]]. The study allowed 18 women and 22 men to sleep only 26 minutes per night over a four-day period. Subjects were given initial [[cognitive tests]] while well-rested, and then were tested again twice a day during the four days of sleep deprivation. On the final test, the average working memory span of the sleep-deprived group had dropped by 38% in comparison to the control group.
 
 
Memory seems to be affected differently by certain stages of sleep such as REM and [[slow-wave sleep]] (SWS). In one study,<ref>cited in {{cite journal|author=Born, J.; Rasch, J.; Gais, S. |year=2006|title=Sleep to remember|journal=Neuroscientist|volume=12|page=410}}</ref> multiple groups of human subjects were used: wake control groups and sleep test groups. Sleep and wake groups were taught a task and were then tested on it, both on early and late nights, with the order of nights balanced across participants. When the subjects' brains were scanned during sleep, [[hypnogram]]s revealed that SWS was the dominant sleep stage during the early night, representing around 23% on average for sleep stage activity. The early-night test group performed 16% better on the [[declarative memory]] test than the control group. During late-night sleep, REM became the most active sleep stage at about 24%, and the late-night test group performed 25% better on the [[procedural memory]] test than the control group. This indicates that procedural memory benefits from late, REM-rich sleep, whereas declarative memory benefits from early, SWS-rich sleep.
 
 
A study conducted by Datta<ref>{{cite journal|author=Datta, S. |year=2000|title=Avoidance task training potentiates phasic pontine-wave density in the rat: A mechanism for sleep-dependent plasticity |journal=The Journal of Neuroscience|volume=20|pages=8607–8613|pmid=11069969|issue=22}}</ref> indirectly supports these results. The subjects chosen were 22 male rats. A box was constructed wherein a single rat could move freely from one end to the other. The bottom of the box was made of a steel grate. A light would shine in the box accompanied by a sound. After a five-second delay, an electrical shock would be applied. Once the shock commenced, the rat could move to the other end of the box, ending the shock immediately. The rat could also use the five-second delay to move to the other end of the box and avoid the shock entirely. The length of the shock never exceeded five seconds. This was repeated 30 times for half the rats. The other half, the control group, was placed in the same trial, but the rats were shocked regardless of their reaction. After each of the training sessions, the rat would be placed in a recording cage for six hours of polygraphic recordings. This process was repeated for three consecutive days. This study found that during the posttrial sleep recording session, rats spent 25.47% more time in REM sleep after learning trials than after control trials. These trials support the results of the Born et al. study, indicating an obvious correlation between REM sleep and [[procedural knowledge]].
 
 
An observation of the Datta study is that the learning group spent 180% more time in SWS than did the control group during the post-trial sleep-recording session. This phenomenon is supported by a study performed by Kudrimoti, Barnes, and McNaughton.<ref>{{cite journal|author=Kudrimoti, H.S.; Barnes, C.A.; McNaughton, B.L.|year=1999 |title=Reactivation of hippocampal cell assemblies: Effects of behavioral state, experience, and EEG dynamics|journal=The Journal of Neuroscience|volume= 19|pages=4090–4101|pmid=10234037|issue=10}}</ref> This study shows that after spatial exploration activity, patterns of [[hippocampal]] place cells are reactivated during SWS following the experiment. In a study by Kudrimoti et al., seven rats were run through a linear track using rewards on either end. The rats would then be placed in the track for 30 minutes to allow them to adjust (PRE), then they ran the track with reward-based training for 30 minutes (RUN), and then they were allowed to rest for 30 minutes. During each of these three periods, [[EEG]] data were collected for information on the rats' sleep stages. Kudrimoti et al. computed the mean firing rates of hippocampal place cells during prebehavior SWS (PRE) and three ten-minute intervals in postbehavior SWS (POST) by averaging across 22 track-running sessions from seven rats. The results showed that ten minutes after the trial RUN session, there was a 12% increase in the mean firing rate of hippocampal place cells from the PRE level; however, after 20 minutes, the mean firing rate returned rapidly toward the PRE level. The elevated firing of hippocampal place cells during SWS after spatial exploration could explain why there were elevated levels of SWS sleep in Datta's study, as it also dealt with a form of spatial exploration.
 
 
A study has also been done involving [[transcranial direct current stimulation|direct current stimulation]] to the [[prefrontal cortex]] to increase the amount of slow oscillations during SWSfe. The direct current stimulation greatly enhanced word-pair retention the following day, giving evidence that SWS plays a large role in the consolidation of episodic memories.<ref>Marshall et al., 2006, as cited in {{cite journal|author=Walker, M.P. |title=The Role of Sleep in Cognition and Emotion|journal= Annals of the New York Academy of Sciences|volume= 1156|year=2009|page=174}}</ref>
 
 
The different studies all suggest that there is a correlation between sleep and the complex functions of memory. Harvard sleep researchers Saper<ref>{{cite journal|doi=10.1038/nature04284|title=Hypothalamic regulation of sleep and circadian rhythms|year=2005|last1=Saper|first1=Clifford B.|last2=Scammell|first2=Thomas E.|last3=Lu|first3=Jun|journal=Nature|volume=437|issue=7063|pages=1257–63|pmid=16251950}}</ref> and [[Robert Stickgold|Stickgold]]<ref>{{cite journal|doi=10.1038/nature04286|title=Sleep-dependent memory consolidation|year=2005|last1=Stickgold|first1=Robert|journal=Nature|volume=437|issue=7063|pages=1272–8|pmid=16251952}}</ref> point out that an essential part of memory and learning consists of nerve cell [[dendrite]]s' sending of information to the cell body to be organized into new neuronal connections. This process demands that no external information is presented to these dendrites, and it is suggested that this may be why it is during sleep that memories and knowledge are solidified and organized.
 
 
===Preservation===
 
The "Preservation and Protection" theory holds that sleep serves an adaptive function. It protects the animal during that portion of the 24-hour day in which being awake, and hence roaming around, would place the individual at greatest risk.<ref name="choi">[http://www.livescience.com/health/090825-why-sleep.html New Theory Questions Why We Sleep] by Charles Q. Choi, LiveScience.com, Aug 25, 2009.</ref> Organisms do not require 24 hours to feed themselves and meet other necessities. From this perspective of adaptation, organisms are safer by staying out of harm's way, where potentially they could be prey to other, stronger organisms. They sleep at times that maximize their safety, given their physical capacities and their habitats.
 
 
This theory fails to explain why the brain disengages from the external environment during normal sleep. However, the brain consumes a large proportion of the body's calories at any one time and preservation of energy could only occur by limiting its sensory inputs. Another argument against the theory is that sleep is not simply a passive consequence of removing the animal from the environment, but is a "drive"; animals alter their behaviors in order to obtain sleep. Therefore, circadian regulation is more than sufficient to explain periods of activity and [[wiktionary:quiescence|quiescence]] that are adaptive to an organism, but the more peculiar specializations of sleep probably serve different and unknown functions. Moreover, the preservation theory needs to explain why carnivores like lions, which are on top of the [[food chain]] and thus have little to fear, sleep the most. It has been suggested that they need to minimize energy expenditure when not hunting.
 
 
Preservation also does not explain why aquatic mammals sleep while moving. Quiescence during these vulnerable hours would do the same and would be more advantageous, because the animal would still be able to respond to environmental challenges like predators, etc. Sleep rebound that occurs after a sleepless night will be maladaptive, but obviously must occur for a reason. A zebra falling asleep the day after it spent the sleeping time running from a lion is more, not less, vulnerable to predation.
 
 
==Dreaming==
 
{{Main|Dream}}
 
[[File:WLA metmuseum Bronze statue of Eros sleeping 7.jpg|thumb|right|Bronze statue of [[Eros]] sleeping, 3rd century BC–early 1st century AD]]
 
Dreaming is the perceived experience of sensory images and sounds during sleep, in a sequence which the dreamer usually perceives more as an apparent participant than as an observer. Dreaming is stimulated by the [[pons]] and mostly occurs during the [[REM phase of sleep]].
 
 
Dreams can also be suppressed or encouraged; taking [[anti-depressants]], Tylenol, Advil, or alcohol is thought to potentially suppress dreams, whereas [[melatonin]] may have the ability to encourage them. <ref>{{Cite article|author=Naiman, Rubin |year=2007|title=How To Interpret Your Dreams|journal= Allure|volume=17|pages=n/a|issue=5}}</ref>
 
 
People have proposed many [[hypotheses]] about the functions of dreaming. [[Sigmund Freud]] postulated that dreams are the symbolic expression of frustrated desires that had been relegated to the [[unconscious mind]], and he used [[dream interpretation]] in the form of [[psychoanalysis]] to uncover these desires. See Freud: [[The Interpretation of Dreams]].
 
 
Freud's work concerns the psychological role of dreams, which does not exclude any physiological role they may have. Recent research<ref>{{Cite news |first=Steve |last=Connor |title=Revealed: why we need a good night's sleep |url=http://www.independent.co.uk/news/science/revealed-why-we-need-a-good-nights-sleep-1661100.html |publisher=The Independent |date=2009-04-03 |accessdate=2010-12-02}}</ref> claims that sleep has the overall role of consolidation and organization of synaptic connections formed during learning and experience. As such, Freud's work is not ruled out. Nevertheless, Freud's research has been expanded on, especially with regard to the organization and consolidation of recent [[memory]].
 
 
Certain processes in the cerebral cortex have been studied by [[Allan Hobson|John Allan Hobson]] and [[Robert McCarley]]. In their [[activation synthesis theory]], for example, they propose that dreams are caused by the [[random]] firing of [[neurons]] in the [[cerebral cortex]] during the REM period. Neatly, this theory helps explain the irrationality of the mind during REM periods, as, according to this theory, the [[forebrain]] then creates a [[narrative|story]] in an attempt to reconcile and make sense of the nonsensical sensory information presented to it.<ref>{{cite journal|author=Hobson, J.A.; McCarley, R. |year=1977|title=The brain as a dream state generator: An activation-synthesis hypothesis of the dream process|journal= American Journal of Psychiatry|volume= 134|pages= 1335–1348|pmid=21570|issue=12}}</ref> Ergo, the odd nature of many dreams.
 
 
==Effect of food and drink on sleep==
 
{{Refimprove section|date=October 2008}}
 
 
===Hypnotics===
 
* [[Nonbenzodiazepine]] hypnotics such as [[eszopiclone]] (Lunesta), [[zaleplon]] (Sonata), and [[zolpidem]] (Ambien) are commonly used as sleep aids prescribed by doctors to treat forms of insomnia. Nonbenzodiazepines are the most commonly prescribed and [[Over-the-counter drug|OTC]] sleep aids used worldwide and have been greatly growing in use since the 1990s. They target the [[GABAA receptor|GABA<sub>A</sub> receptor]].
 
* [[Benzodiazepine]]s target the [[GABAA receptor|GABA<sub>A</sub> receptor]] also, and as such, they are commonly used sleep aids as well, though benzodiazepines have been found to decrease [[REM sleep]].<ref name="sleep_medicine_a04">{{Cite book |last=Lee-chiong |first=Teofilo |title=Sleep Medicine: Essentials and Review |date=24 April 2008 |publisher=[[Oxford University Press]], USA |url=http://books.google.com/?id=s1F_DEbRNMcC&pg=PT52 |isbn=0-19-530659-7 |page=52}}</ref>
 
* [[Antihistamine]]s, such as [[diphenhydramine]] (Benadryl) and [[doxylamine]] (found in various OTC medicines, such as [[NyQuil]])
 
* [[Alcoholic beverage|Alcohol]] – Often, people start drinking alcohol in order to get to sleep (alcohol is initially a sedative and will cause [[somnolence]], encouraging sleep).<ref>[http://www.sleepdex.org/alcohol1.htm Alcohol and Sleep]. Sleepdex.org. Retrieved on 2011-12-01.</ref> However, being addicted to alcohol can lead to disrupted sleep, because alcohol has a [[rebound effect]] later in the night. As a result, there is strong evidence linking alcoholism and forms of insomnia.<ref>[http://alcoholism.about.com/cs/alerts/l/blnaa41.htm Alcohol and Sleep]. Alcoholism.about.com (2011-01-10). Retrieved on 2011-12-01.</ref> Alcohol also reduces [[REM sleep]].<ref name="sleep_medicine_a04"/>
 
* [[Barbiturate]]s cause drowsiness and have actions similar to alcohol in that they have a [[rebound effect]] and inhibit REM sleep, so they are not used as a long-term sleep aid.<ref>[http://www.sleepdex.org/barbituates.htm Sleep Medications: Barbituates]. Sleepdex.org. Retrieved on 2011-12-01.</ref>
 
* [[Melatonin]] is a naturally occurring hormone that regulates sleepiness. It is made in the brain, where tryptophan is converted into serotonin and then into melatonin, which is released at night by the [[pineal gland]] to induce and maintain sleep. Melatonin supplementation may be used as a sleep aid, both as a [[hypnotic]] and as a [[chronobiotic]] (see [[phase response curve]], PRC).
 
* [[Siesta]] and the "post-lunch dip" – Many people have a temporary drop in alertness in the early afternoon, commonly known as the "post-lunch dip." While a large meal can make a person feel sleepy, the post-lunch dip is mostly an effect of the [[circadian rhythm|biological clock]]. People naturally feel most sleepy (have the greatest "drive for sleep") at two times of the day about 12 hours apart—for example, at 2:00 a.m. and 2:00 p.m. At those two times, the body clock "kicks in." At about 2 p.m. (14:00), it overrides the homeostatic buildup of sleep debt, allowing several more hours of wakefulness. At about 2 a.m. (02:00), with the daily sleep debt paid off, it "kicks in" again to ensure a few more hours of sleep.
 
* [[Tryptophan]] – The amino acid tryptophan is a building block of proteins. It has been claimed to contribute to sleepiness, since it is a precursor of the neurotransmitter serotonin, involved in sleep regulation. However, no solid data have ever linked modest dietary changes in tryptophan to changes in sleep.
 
* [[Cannabis (drug)|Marijuana]] – Some people use marijuana to induce sleepiness. Users often report relaxation and drowsiness. It has been shown that [[Tetrahydrocannabinol]], the principal psychoactive constituent in marijuana, reduces the amount of REM sleep.<ref>[http://www.psychologytoday.com/blog/the-teenage-mind/200906/marijuana-sleep-and-dreams Marijuana, Sleep and Dreams]. psychologytoday.com. Retrieved on 2012-02-10.</ref> Frequent users often report being unable to recall their dreams.
 
 
===Stimulants===
 
* [[Amphetamine]] ([[dextroamphetamine]], and a related, slightly more powerful drug [[methamphetamine]], etc.) are used to treat [[narcolepsy]]. Their most common effects are anxiety, insomnia, stimulation, increased alertness, and decreased hunger.
 
* [[Caffeine]] is a [[stimulant]] that works by slowing the action of the hormones in the brain that cause [[somnolence]], particularly by acting as an [[antagonist]] at [[adenosine]] receptors. Effective dosage is individual, in part dependent on prior usage. It can cause a rapid reduction in alertness as it wears off.
 
* [[Cocaine]] and [[crack cocaine]] – Studies on cocaine have shown its effects to be mediated through the circadian rhythm system.<ref>{{cite journal |author=Abarca C, Albrecht U, Spanagel R |title=Cocaine sensitization and reward are under the influence of circadian genes and rhythm |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=99 |issue=13 |pages=9026–30 |date=June 2002 |pmid=12084940 |pmc=124417 |doi=10.1073/pnas.142039099}}</ref> This may be related to the onset of [[hypersomnia]] (oversleeping) in regard to "cocaine-induced [[sleep disorder]]."<ref>[http://www.mindsite.com/dsm_iv/primary_hypersomnia Primary hypersomnia: Diagnostic Features]{{dead link|date=December 2011}}</ref>
 
* [[MDMA]], including similar drugs like [[3,4-Methylenedioxyamphetamine|MDA]], [[MMDA (psychedelic)|MMDA]], or [[Methylenedioxymethcathinone|bk-MDMA]] – The class of drugs called [[empathogen-entactogen]]s keep users awake with intense euphoria. Commonly known as "ecstasy."
 
* [[Methylphenidate]] – Commonly known by the brand names ''Ritalin'' and ''Concerta'', methylphenidate is similar in action to amphetamine and cocaine; its chemical composition more closely resembles that of cocaine.
 
* [[Tobacco]] – Tobacco has been found not only to disrupt but also to reduce total sleep time. In studies, users have described more daytime drowsiness than nonsmokers.<ref>[http://www.sleep.com/content/causes-of-sleep-deprivation/ Causes of Sleep Deprivation] Sleep.com.</ref>
 
* Other [[analeptic]] drugs like [[Modafinil]] and [[Armodafinil]] are prescribed to treat narcolepsy, [[hypersomnia]], [[shift work sleep disorder]], and other conditions causing [[Excessive Daytime Sleepiness]]. The precise mechanism of these CNS stimulants is not known, but they have been shown to increase both the release of monoamines and levels of hypothalamic histamine, thereby promoting wakefulness.
 
 
==Insomnia==
 
[[Insomnia]] is a general term describing difficulty falling asleep and staying asleep. Insomnia can have many different causes, including psychological stress, a poor sleep environment, an inconsistent sleep schedule, or excessive mental or physical stimulation in the hours before bedtime. Insomnia is often treated through behavioral changes like keeping a regular sleep schedule, avoiding stimulating or stressful activities before bedtime, and cutting down on stimulants such as caffeine. Patients are often counseled to improve their sleep environment by installing heavy drapes to shut out all sunlight, and keeping computers, televisions and work materials out of the sleeping area.
 
 
A 2010 review of published scientific research suggested that exercise generally improves sleep for most people, and helps sleep disorders such as insomnia. The optimum time to exercise ''may'' be 4 to 8 hours before bedtime, though exercise at any time of day is beneficial, with the exception of heavy exercise taken shortly before bedtime, which may disturb sleep. However there is insufficient evidence to draw detailed conclusions about the relationship between exercise and sleep.<ref>Buman, M.P. and King, A.C.: "Exercise as a Treatment to Enhance Sleep", ''[[American Journal of Lifestyle Medicine]]'', Nov–Dec 2010.</ref>
 
 
Sleeping medications such as [[Ambien]] and [[Lunesta]] are an increasingly popular treatment for insomnia, and have become a major source of revenue for drug companies. Although these nonbenzodiazepine medications are generally believed to be better and safer than earlier generations of sedatives, they have still generated some controversy and discussion regarding side-effects.
 
 
[[White noise]] appears to be a promising treatment for [[insomnia]].<ref>{{cite journal |author=López HH, Bracha AS, Bracha HS |title=Evidence based complementary intervention for insomnia |journal=Hawaii Med J |volume=61 |issue=9 |pages=192, 213 |year=2002 |pmid=12422383 |url=http://cogprints.org/5032/1/2002_H.M.J_White-noise_for_PTSD.pdf}}</ref>
 
 
==Obstructive sleep apnea==
 
[[Obstructive sleep apnea]] is a condition in which major pauses in breathing occur during sleep, disrupting the normal progression of sleep and often causing other more severe health problems. Apneas occur when the muscles around the patient's airway relax during sleep, causing the airway to collapse and block the intake of oxygen. As oxygen levels in the blood drop, the patient then comes out of deep sleep in order to resume breathing. When several of these episodes occur per hour, sleep apnea rises to a level of seriousness that may require treatment.
 
 
Diagnosing sleep apnea usually requires a professional sleep study performed in a sleep clinic, because the episodes of wakefulness caused by the disorder are extremely brief and patients usually do not remember experiencing them. Instead, many patients simply feel tired after getting several hours of sleep and have no idea why. Major risk factors for sleep apnea include chronic fatigue, old age, obesity and snoring.
 
 
==Other sleep disorders==
 
[[File:People sleeping within an airplane during a long, night flight..jpg|thumb|People sleeping on a train at night]]
 
 
[[Sleep disorder]]s include [[narcolepsy]], [[nocturnal myoclonus|periodic limb movement disorder]] (PLMD), [[restless leg syndrome]] (RLS), and the [[circadian rhythm sleep disorder]]s. [[Fatal familial insomnia]], or FFI, is an extremely rare genetic disease with no known treatment or cure, is characterized by increasing insomnia as one of its symptoms; ultimately sufferers of the disease stop sleeping entirely, before dying of the disease.<ref name="NatGeoMay2010"/>
 
 
[[Old age|Older people]] are more easily awakened by disturbances in the environment<ref>[http://www.npr.org/templates/transcript/transcript.php?storyId=111415462 How Aging Changes Sleep Patterns] by Allison Aubrey. Morning Edition, 3 Aug 2009.</ref> and may to some degree lose the ability to consolidate sleep.
 
 
==Anthropology of sleep==
 
Research suggests that sleep patterns vary significantly across cultures.<ref name=Worthman>{{cite book |author=Carol M. Worthman and Melissa K. Melby |title=A comparative developmental ecology |chapter=6. Toward a comparative developmental ecology of human sleep |chapterurl=http://webdrive.service.emory.edu/groups/research/lchb/PUBLICATIONS%20Worthman/PUBLICATIONS%20CMW%202002/Ecology%20of%20Human%20sleep.pdf |format=PDF |publisher=Emory University}}</ref><ref>[http://www.sciencenews.org/pages/sn_arc99/9_25_99/bob2.htm Slumber's Unexplored Landscape]. Science News Online (1999-09-25). Retrieved on 2011-12-01.</ref> The most striking differences are between societies that have plentiful sources of artificial light and ones that do not.<ref name=Worthman/> The primary difference appears to be that pre-light cultures have more broken-up sleep patterns.<ref name=Worthman/> For example, people might go to sleep far sooner after the sun sets, but then wake up several times throughout the night, punctuating their sleep with periods of wakefulness, perhaps lasting several hours.<ref name=Worthman/> The boundaries between sleeping and waking are blurred in these societies.<ref name=Worthman/> Some observers believe that nighttime sleep in these societies is most often split into two main periods, the first characterized primarily by deep sleep and the second by REM sleep.<ref name=Worthman/>
 
 
Some societies display a fragmented sleep pattern in which people sleep at all times of the day and night for shorter periods. In many [[nomadic]] or [[hunter-gatherer]] societies, people will sleep on and off throughout the day or night depending on what is happening.<ref name=Worthman/> Plentiful [[artificial light]] has been available in the industrialized West since at least the mid-19th century, and sleep patterns have changed significantly everywhere that lighting has been introduced.<ref name=Worthman/> In general, people sleep in a more concentrated burst through the night, going to sleep much later, although this is not always true.<ref name=Worthman/> Historian Roger Ekrich thinks that the traditional pattern of "[[segmented sleep]]" as it is called began to disappear among the urban upper class in Europe in the late 17th century and the change spread over the next 200 years; by the 1920s "the idea of a first and second sleep had receded entirely from our social consciousness."<ref name=bbc>{{cite news |last=Hegarty|first=Stephanie|title=The myth of the eight-hour sleep|url=http://www.bbc.co.uk/news/magazine-16964783|accessdate=22 February 2012|newspaper=BBC News|date=22 February 2012}}</ref> Ekrich attributes the change to increases in "street lighting, domestic lighting and a surge in coffee houses," which slowly made nighttime a legitimate time for activity, decreasing the time available for rest.<ref name=bbc/>
 
 
In some societies, people generally sleep with at least one other person (sometimes many) or with animals. In other cultures, people rarely sleep with anyone but a most intimate relation, such as a spouse. In almost all societies, sleeping partners are strongly regulated by social standards. For example, people might only sleep with their [[immediate family]], [[extended family]], spouses, their children, children of a certain age, children of specific gender, peers of a certain gender, friends, peers of equal social rank, or with no one at all. Sleep may be an actively social time, depending on the sleep groupings, with no constraints on noise or activity.<ref name=Worthman/>
 
 
People sleep in a variety of locations. Some sleep directly on the ground; others on a skin or blanket; others sleep on platforms or [[bed]]s. Some sleep with blankets, some with pillows, some with simple headrests, some with no head support. These choices are shaped by a variety of factors, such as climate, protection from predators, housing type, technology, personal preference, and the incidence of pests.<ref name="Worthman"/>
 
 
==Sleep in non-humans==
 
[[Image:Japanese Macaques sleeping.JPG|thumb|right|Sleeping [[Japanese Macaque]]s]]
 
{{Main|Sleep (non-human)}}
 
Neurological sleep states can be difficult to detect in some animals. In these cases, sleep may be defined using behavioral characteristics such as minimal movement, postures typical for the species, and reduced responsiveness to external stimulation. Sleep is quickly reversible, as opposed to hibernation or [[coma]], and sleep deprivation is followed by longer or deeper rebound sleep. Herbivores, who require a long waking period to gather and consume their diet, typically sleep less each day than similarly sized carnivores, who might well consume several days' supply of meat in a sitting.
 
 
Horses and other herbivorous [[ungulates]] can sleep while standing, but must necessarily lie down for REM sleep (which causes muscular atony) for short periods. Giraffes, for example, only need to lie down for REM sleep for a few minutes at a time. Bats sleep while hanging upside down. Some aquatic mammals and some birds can sleep with one half of the brain while the other half is awake, so-called [[unihemispheric slow-wave sleep]].<ref>{{cite journal |author=Mukhametova, LM |coauthors=Supina, AY; Polyakovaa, IG |title=Interhemispheric asymmetry of the electroencephalographic sleep patterns in dolphins |journal=Brain Research |volume=134 |issue=3 |pages=581–584 |date=1977-10-14 |doi=10.1016/0006-8993(77)90835-6 |pmid=902119}}</ref> Birds and mammals have cycles of non-REM and REM sleep (as described above for humans), though birds' cycles are much shorter and they do not lose muscle tone (go limp) to the extent that most mammals do.
 
 
Many mammals sleep for a large proportion of each 24-hour period when they are very young.<ref>{{cite web |url=http://www.madsci.org/posts/archives/2000-08/965504574.Zo.r.html |title=Re: Are there animals who don't sleep or that sleep very little? |accessdate=2008-01-25 |last=Faraco |first=Juliette |date=2000-08-01 |work=MadSci Network: Zoology}}</ref> However, [[killer whale]]s and some other [[dolphin]]s do not sleep during the first month of life.<ref> The giraffe only sleeps 2 hours a day in about 5-15 minute sessions. Koalas are the longest sleeping-mammals, about 20-22 hours a day.[http://www.livescience.com/animals/050629_sleepless_sea.html Insomnia Mania: Newborn Mammals Don't Sleep for a Month]. LiveScience.com</ref> Such differences may be explained by the ability of land-mammal newborns to be easily protected by parents while sleeping, while marine animals must, even while very young, be more continuously vigilant for predators.
 
 
==See also==
 
{{Div col|cols=2}}
 
* [[Cortisol awakening response]]
 
* [[Microsleep]]
 
* [[Morvan's syndrome]]
 
* [[Polyphasic sleep]]
 
* [[Power nap]]
 
* [[Segmented sleep]]
 
* [[Sleep architecture]]
 
* [[Sleep medicine]]
 
* [[Somnology]]
 
* [[Sudden infant death syndrome]]
 
* [[Sudden unexpected death syndrome]]
 
* [[Ultradian]] and [[circadian rhythm]]s.
 
{{Div col end}}
 
 
===Positions, practices, and rituals===
 
{{Div col|cols=2}}
 
* [[Sleeping positions]]
 
* [[Co-sleeping]]
 
* [[Hypnosis]]
 
* [[Meditation]]
 
* [[Neutral spine]]
 
* [[Sleep hygiene]]
 
* [[Yoga-nidra]]
 
* [[World Sleep Day]]
 
{{Div col end}}
 
 
==References==
 
{{Reflist|35em|refs=
 
<ref name=Morrissey>{{cite journal |author=Morrissey M, Duntley S, Anch A, Nonneman R |title=Active sleep and its role in the prevention of apoptosis in the developing brain |journal=Med Hypotheses |volume=62 |issue=6 |pages= 876–9 |year=2004 |pmid=15142640 |doi=10.1016/j.mehy.2004.01.014}}</ref>
 
 
<ref name=Naska>{{cite journal|url=http://archinte.ama-assn.org/cgi/content/full/167/3/296|author=Naska, A., Oikonomou, E., Trichopoulou, A., Psaltopoulou, T. and Trichopoulos, D. |year=2007|title= Siesta in healthy adults and coronary mortality in the general population|journal= Archives of Internal Medicine|volume= 167|pages= 296–301|doi=10.1001/archinte.167.3.296|pmid=17296887|issue=3}}</ref>
 
 
<ref name=Zaregarizi1>{{cite doi|10.1152/japplphysiol.00474.2007}}</ref>
 
<ref name=Zaregarizi2>{{cite book|title=Effects of Exercise & Daytime Sleep on Human Haemodynamics: With Focus on Changes in Cardiovascular Function during Daytime Sleep Onset|isbn=978-3-8484-1726-1|author=MohammadReza Zaregarizi}}</ref>
 
}}
 
 
==External links==
 
{{Commons category|Sleeping|Sleep}}
 
{{Wiktionary}}
 
* [http://www.nhlbi.nih.gov/about/ncsdr/index.htm NIH.gov], National Center on Sleep Disorders Research
 
*[http://www.sleepfoundation.org/article/sleep-topics/school-start-time-and-sleep/ National Sleep Foundation], information on School Start Time and Sleep at [[National Sleep Foundation]]
 
*[http://healthysleep.med.harvard.edu/ HealthySleep.med.harvard.edu], from the Division of Sleep Medicine at [[Harvard Medical School]] and [[WGBH-TV|WGBH]] Educational Foundation
 
*[http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0060216&ct=1 PLOSjournals.org], Is Sleep Essential? by Chiara Cirelli and Giulio Tononi, from the [[Public Library of Science]], Biology
 
*[http://www.nytimes.com/2012/09/23/opinion/sunday/rethinking-sleep.html?pagewanted=1&_r=0&ref=general&src=me New York Times, 2012; ''"Rethinking Sleep"'']
 
{{Neuroscience}}
 
{{Psychophysiology}}
 
{{SleepSeries2}}
 
[[Category:Sleep| ]]
 
[[Category:Neuroscience]]
 
[[Category:Unsolved problems in neuroscience]]
 
 
{{Link GA|de}}
 
 
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Reason: ANN scored at 0.973666
Reporter Information
Reporter: mnnqucmt (anonymous)
Date: Sunday, the 30th of August 2015 at 02:17:35 PM
Status: Reported
Sunday, the 30th of August 2015 at 02:17:35 PM #100915
mnnqucmt (anonymous)

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