Sleep is a state of reduced mental and physical activity in whichconsciousness is altered and certainsensory activity is inhibited. During sleep, there is a marked decrease in muscle activity and interactions with the surrounding environment. While sleep differs fromwakefulness in terms of the ability to react tostimuli, it still involves activebrain patterns, making it more reactive than acoma ordisorders of consciousness.[1]
Sleep occurs inrepeating periods, during which the body alternates between two distinct modes:rapid eye movement sleep (REM) andnon-REM sleep. Although REM stands for "rapid eye movement", this mode of sleep has many other aspects, including virtualparalysis of the body.[2]Dreams are a succession of images, ideas, emotions, and sensations that usually occur involuntarily in themind during certain stages of sleep.
During sleep, most of thebody's systems are in ananabolic state, helping to restore theimmune,nervous,skeletal, andmuscular systems;[3] these are vital processes that maintainmood,memory, andcognitive function, and play a large role in the function of theendocrine andimmune systems.[4] The internalcircadian clock promotes sleep daily atnight, when it is dark. The diverse purposes andmechanisms of sleep are the subject of substantial ongoing research.[5] Sleep is a highlyconserved behavior across animal evolution,[6] likely going back hundreds of millions of years,[7] and originating as a means for the brain to cleanse itself of waste products.[8] In a major breakthrough, researchers have found that cleansing, including the removal ofamyloid, may be a core purpose of sleep.[9]
The most pronounced physiological changes in sleep occur in the brain.[12] The brain uses significantly less energy during sleep than it does when awake, especially during non-REM sleep. In areas with reduced activity, the brain restores its supply ofadenosine triphosphate (ATP), the molecule used for short-term storage and transport of energy.[13] In quiet waking, the brain is responsible for 20% of the body's energy use, thus this reduction has a noticeable effect on overall energy consumption.[14]
Sleep increases thesensory threshold. In other words, sleeping persons perceive fewer stimuli, but can generally still respond to loud noises and other salient sensory events.[14][12]
The electrical activity seen on an EEG represents brain waves. The amplitude of EEG waves at a particular frequency corresponds to various points in the sleep-wake cycle, such as being asleep, being awake, or falling asleep.[18] Alpha, beta, theta, gamma, and delta waves are all seen in the different stages of sleep. Each waveform maintains a different frequency and amplitude. Alpha waves are seen when a person is in a resting state, but is still fully conscious. Their eyes may be closed and all of their body is resting and relatively still, where the body is starting to slow down. Beta waves take over alpha waves when a person is at attention, as they might be completing a task or concentrating on something. Beta waves consist of the highest of frequencies and the lowest of amplitude, and occur when a person is fully alert. Gamma waves are seen when a person is highly focused on a task or using all their concentration. Theta waves occur during the period of a person being awake, and they continue to transition into Stage 1 of sleep and in stage 2. Delta waves are seen in stages 3 and 4 of sleep when a person is in their deepest of sleep.[19]
Non-REM and REM sleep
Sleep is divided into two broad types:non-rapid eye movement (non-REM or NREM) sleep andrapid eye movement (REM) sleep. Non-REM and REM sleep are so different that physiologists identify them as distinct behavioral states. Non-REM sleep occurs first and after a transitional period is calledslow-wave sleep or deep sleep. During this phase, body temperature and heart rate fall, and the brain uses less energy.[12] REM sleep, also known as paradoxical sleep, represents a smaller portion of total sleep time. It is the main occasion for dreams (ornightmares), and is associated with desynchronized and fast brain waves, eye movements, loss of muscle tone,[20] and suspension ofhomeostasis.[21]
Thesleep cycle of alternate NREM and REM sleep takes an average of 90 minutes, occurring 4–6 times in a good night's sleep.[17][22] TheAmerican Academy of Sleep Medicine (AASM) divides NREM into three stages: N1, N2, and N3, the last of which is also called delta sleep orslow-wave sleep.[23] The whole period normally proceeds in the order: N1 → N2 → N3 → N2 → REM. REM sleep occurs as a person returns to stage 2 or 1 from a deep sleep.[20] 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.[17]
"The Awakening", an illustration to writing byLeo Tolstoy
Awakening can mean the end of sleep, or simply a moment to survey the environment and readjust body position before falling back asleep. Sleepers typically awaken soon after the end of a REM phase or sometimes in the middle of REM. Internalcircadian indicators, along with a successful reduction of homeostatic sleep need, typically bring about awakening and the end of the sleep cycle.[24] Awakening involves heightened electrical activation in the brain, beginning with thethalamus and spreading throughout thecortex.[24]
On a typical night of sleep, there is not much time that is spent in the waking state. In various sleep studies that have been conducted using the electroencephalography, it has been found that females are awake for 0–1% during their nightly sleep while males are awake for 0–2% during that time. In adults, wakefulness increases, especially in later cycles. One study found 3% awake time in the first ninety-minute sleep cycle, 8% in the second, 10% in the third, 12% in the fourth, and 13–14% in the fifth. Most of this awake time occurred shortly after REM sleep.[24]
Today, many humans wake up with analarm clock;[25] however, people can also reliably wake themselves up at a specific time with no need for an alarm.[24] Many sleep quite differently on workdays versus days off, a pattern which can lead to chronic circadian desynchronization.[26][25] Many people regularly look at television and other screens before going to bed, a factor which may exacerbate disruption of the circadian cycle.[27][28] Scientific studies on sleep have shown that sleep stage at awakening is an important factor in amplifyingsleep inertia.[29]
Determinants ofalertness after waking up include quantity/quality of the sleep, physical activity the day prior, a carbohydrate-rich breakfast, and a lowblood glucose response to it.[30]
Timing
Sleep timing is controlled by thecircadian clock (Process C), sleep-wakehomeostasis (Process S), and to some extent by the individual will.
Sleep timing depends greatly onhormonal signals from the circadian clock, or Process C, a complex neurochemical system which uses signals from an organism's environment to recreate an internal day–night rhythm. Process C counteracts the homeostatic drive for sleep during the day (indiurnal animals) and augments it at night.[31][26] Thesuprachiasmatic nucleus (SCN), a brain area directly above theoptic chiasm, is presently considered the most important nexus for this process; however, secondary clock systems have been found throughout the body.
An organism whose circadian clock exhibits a regular rhythm corresponding to outside signals is said to beentrained; an entrained rhythm persists even if the outside signals suddenly disappear. If an entrained human is isolated in a bunker with constant light or darkness, he or she will continue to experience rhythmic increases and decreases of body temperature and melatonin, on a period that slightly exceeds 24 hours. Scientists refer to such conditions asfree-running of the circadian rhythm. Under natural conditions, light signals regularly adjust this period downward, so that it corresponds better with the exact 24 hours of an Earth day.[25][32][33]
The circadian clock exerts constant influence on the body, affectingsinusoidal oscillation ofbody temperature between roughly 36.2 °C and 37.2 °C.[33][34] The suprachiasmatic nucleus itself shows conspicuous oscillation activity, which intensifies during subjective day (i.e., the part of the rhythm corresponding with daytime, whether accurately or not) and drops to almost nothing during subjective night.[35] The circadian pacemaker in the suprachiasmatic nucleus has a direct neural connection to thepineal gland, which releases the hormonemelatonin at night.[35]Cortisol levels typically rise throughout the night,peak in the awakening hours, and diminish during the day.[15][36] Circadianprolactin secretion begins in the late afternoon, especially in women, and is subsequently augmented by sleep-induced secretion, to peak in the middle of the night. Circadian rhythm exerts some influence on the nighttime secretion of growth hormone.[15]
The circadian rhythm influences the ideal timing of a restorative sleep episode.[25][37] Sleepiness increases during the night. REM sleep occurs more during body temperature minimum within the circadian cycle, whereasslow-wave sleep can occur more independently of circadian time.[33]
The internal circadian clock is profoundly influenced by changes in light, since these are its main clues about what time it is. Exposure to even small amounts of light during the night can suppress melatonin secretion, and increase body temperature and wakefulness. Short pulses of light, atthe right moment in the circadian cycle, can significantly 'reset' the internal clock.[34] Blue light, in particular, exerts the strongest effect,[26] leading to concerns thatuse of a screen before bed may interfere with sleep.[27]
Modern humans often find themselves desynchronized from their internal circadian clock, due to the requirements of work (especiallynight shifts), long-distance travel, and the influence of universal indoor lighting.[33] Even if they have sleep debt, or feel sleepy, people can have difficulty staying asleep at the peak of their circadian cycle. Conversely, they can have difficulty waking up in the trough of the cycle.[24] A healthy young adult entrained to the sun will (during most of the year) fall asleep a few hours after sunset, experience body temperature minimum at 6 a.m., and wake up a few hours after sunrise.[33]
Generally speaking, the longer an organism is awake, the more it feels a need to sleep ("sleep debt"). This driver of sleep is referred to asProcess S. The balance between sleeping and waking is regulated by a process calledhomeostasis. Induced or perceived lack of sleep is calledsleep deprivation.
Sleep deprivation tends to cause slower brain waves in thefrontal cortex, shortened attention span, higher anxiety, impaired memory, and a grouchymood. Conversely, a well-rested organism tends to have improved memory and mood.[39] Neurophysiological and functionalimaging studies have demonstrated that frontal regions of the brain are particularly responsive to homeostatic sleep pressure.[40]
There is disagreement on how much sleep debt is possible to accumulate, and whether sleep debt is accumulated against an individual's average sleep or some other benchmark. It is also unclear whether the prevalence of sleep debt among adults has changed appreciably in theindustrialized world in recent decades. Sleep debt does show some evidence of being cumulative. Subjectively, however, humans seem to reach maximum sleepiness 30 hours after waking.[33] It is likely that inWestern societies, children are sleeping less than they previously have.[41]
One neurochemical indicator of sleep debt isadenosine, a neurotransmitter that inhibits many of the bodily processes associated with wakefulness. Adenosine levels increase in the cortex and basal forebrain during prolonged wakefulness, and decrease during the sleep-recovery period, potentially acting as a homeostatic regulator of sleep.[42][43]Coffee, tea, and other sources ofcaffeine temporarily block the effect of adenosine, prolong sleep latency, and reduce total sleep time and quality.[44]
Social timing
Humans are also influenced by aspects ofsocial time, such as the hours when other people are awake, the hours when work is required, the time on clocks, etc.Time zones, standard times used to unify the timing for people in the same area, correspond only approximately to the natural rising and setting of the sun. An extreme example of the approximate nature of time zones is China, a country which used to span five time zones and nowofficially uses only one (UTC+8).[25]
Distribution
Inpolyphasic sleep, an organism sleeps several times in a 24-hour cycle, whereas in monophasic sleep this occurs all at once. Under experimental conditions, humans tend to alternate more frequently between sleep and wakefulness (i.e., exhibit more polyphasic sleep) if they have nothing better to do.[33] Given a 14-hour period of darkness in experimental conditions, humans tended towards bimodal sleep, with two sleep periods concentrated at the beginning and at the end of the dark time. Bimodal sleep in humans was more common before theIndustrial Revolution.[36]
Different characteristic sleep patterns, such as the familiarly so-called "early bird" and "night owl", are calledchronotypes. Genetics and sex have some influence on chronotype, but so do habits. Chronotype is also liable to change over the course of a person's lifetime. Seven-year-olds are better disposed to wake up early in the morning than are fifteen-year-olds.[26][25] Chronotypes far outside the normal range are called circadian rhythm sleep disorders.[45]
Naps are short periods of sleep that one might take during the daytime, often in order to get the necessary amount of rest. Napping is often associated with childhood, but around one-third of American adults partake in it daily. The optimal nap duration is around 10–20 minutes, as researchers have proven that it takes at least 30 minutes to enter slow-wave sleep, the deepest period of sleep.[46] Napping too long and entering the slow wave cycles can make it difficult to awake from the nap and leave one feeling unrested. This period of drowsiness is calledsleep inertia.
Thesiesta habit has recently been associated with a 37% lower coronary mortality, possibly due to reduced cardiovascular stress mediated by daytime sleep.[47] Short naps at mid-day and mild evening exercise were found to be effective for improved sleep, cognitive tasks, and mental health in elderly people.[48]
Genetics
Monozygotic (identical) but not dizygotic (fraternal) twins tend to have similar sleep habits. Neurotransmitters, molecules whose production can be traced to specific genes, are one genetic influence on sleep that can be analyzed. The circadian clock has its own set of genes.[49] Genes which may influence sleep includeABCC9,DEC2,Dopamine receptor D2[50] and variants nearPAX 8 andVRK2.[51] While the latter have been found in aGWAS study that primarily detects correlations (but not necessarily causation), other genes have been shown to have a more direct effect. For instance, mice lackingdihydropyrimidine dehydrogenase (Dpyd) had 78.4 min less sleep during the lights-off period than wild-type mice. Dpyd encodes the rate-limitingenzyme in the metabolic pathway that catabolizesuracil andthymidine to β-alanine, an inhibitoryneurotransmitter. This also supports the role of β-alanine as a neurotransmitter that promotes sleep in mice.[52]
This condition is inherited as an autosomal dominant trait.
Familial natural short sleep is a rare, genetic, typicallyinherited trait where an individual sleeps for fewer hours than average without suffering from daytime sleepiness or other consequences ofsleep deprivation. This process is entirely natural in this kind of individual, and it is caused by certain genetic mutations.[53][54][55][56] A person with this trait is known as a "natural short sleeper".[57]
This condition is not to be confused with intentional sleep deprivation, which leaves symptoms such asirritability or temporarily impaired cognitive abilities in people who are predisposed to sleep a normal amount of time but not in people with FNSS.[58][59][60]
This sleep type is not considered to be a genetic disorder nor are there any known harmful effects to overall health associated with it; therefore it is considered to be a genetic, benign trait.[61]
The quality of sleep may be evaluated from an objective and a subjective point of view. Objective sleep quality refers to how difficult it is for a person to fall asleep and remain in a sleeping state, and how many times they wake up during a single night. Poor sleep quality disrupts the cycle of transition between the different stages of sleep.[63] Subjective sleep quality in turn refers to a sense of being rested and regenerated after awaking from sleep. A study by A. Harvey et al. (2002) found that insomniacs were more demanding in their evaluations of sleep quality than individuals who had no sleep problems.[64]
Homeostatic sleep propensity (the need for sleep as a function of time elapsed since the last adequate sleep episode) must be balanced against the circadian element for satisfactory sleep.[65][66] Along with corresponding messages from the circadian clock, this tells the body it needs to sleep.[67] The timing is correct when the following two circadian markers occur after the middle of the sleep episode and before awakening:[37] maximum concentration of the hormone melatonin, and minimum core body temperature.
Ideal duration
Centers for Disease Control and Prevention (CDC) recommendations for sleep needed decrease with age.[68]The main health effects ofsleep deprivation,[69] indicating impairment of normal maintenance by sleep
Human sleep-needs vary by age and amongst individuals;[70] sleep is considered to be adequate when there is no daytime sleepiness or dysfunction.[71] Moreover, self-reported sleep duration is only moderately correlated with actual sleep time as measured byactigraphy,[72] and those affected withsleep state misperception may typically report having slept only four hours despite having slept a full eight hours.[73][74][75]
Researchers have found that sleeping 6–7 hours each night correlates with longevity and cardiac health in humans, though many underlying factors may be involved in the causality behind this relationship.[76][77][78][79][51][80][81]
Sleep difficulties are furthermore associated with psychiatric disorders such asdepression,alcoholism, andbipolar disorder.[82] Up to 90 percent of adults with depression are found to have sleep difficulties. Dysregulation detected by 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.[83]
Sleep duration can also vary according to season. Up to 90% of people report longer sleep duration in winter, which may lead to more pronouncedseasonal affective disorder.[84][85]
Bronze statue ofEros sleeping, 3rd century BC–early 1st century AD
By the time infants reach the age of two, their brain size has reached 90 percent of an adult-sized brain;[86] a majority of this brain growth has occurred during the period of life with the highest rate of sleep. The hours that children spend asleep influence their ability to perform on cognitive tasks.[87][88] Children who sleep through the night and have few night waking episodes have higher cognitive attainments and easier temperaments than other children.[88][89][90]
Sleep also influences language development. To test this, researchers taught infants a faux language and observed their recollection of the rules for that language.[91] Infants who slept within four hours of learning the language could remember the language rules better, while infants who stayed awake longer did not recall those rules as well. There is also a relationship between infants' vocabulary and sleeping: infants who sleep longer at night at 12 months have better vocabularies at 26 months.[90]
Children can greatly benefit from a structured bedtime routine. This can look differently among families, but will generally consist of a set of rituals such as reading a bedtime story, a bath, brushing teeth, and can also include a show of affection from the parent to the child such as a hug or kiss before bed. A bedtime routine will also include a consistent time that the child is expected to be in bed ready for sleep. Having a reliable bedtime routine can help improve a child's quality of sleep as well as prepare them to make and keep healthysleep hygiene habits in the future.[92]
Recommended duration
World War II poster issued by the US government
Children need many hours of sleep per day in order to develop and function properly: up to 18 hours fornewborn babies, with a declining rate as a child ages.[67] Early in 2015, after a two-year study,[93] theNational Sleep Foundation in the US announced newly revised recommendations as shown in the table below.
The sleeping brain has been shown to remove metabolic end products at a faster rate than during an awake state, by increasing the flow of cerebrospinal fluid during sleep.[94] The mechanism for this removal appears to be theglymphatic system, a system that does for the brain what the lymphatic system does for the body.[94][8] Further research has shown that the glymphatic system is driven by pulses of hormones that in turn create surges in blood flow that cause the cerebrospinal fluid to flow, carrying away metabolites.[95]
Sleep may facilitate the synthesis of molecules that help repair and protect the brain from metabolic end products generated during waking.[96]Anabolic hormones, such asgrowth hormones, are secreted preferentially during sleep. The brain concentration ofglycogen increases during sleep, and is depleted through metabolism during wakefulness.[97]
The human organism physically restores itself during sleep, occurring mostly duringslow-wave sleep during which body temperature, heart rate, and brain oxygen consumption decrease. In both the brain and body, the reduced rate ofmetabolism enables countervailing restorative processes.[97] While the body benefits from sleep, the brain actually requires sleep for restoration, whereas these processes can take place during quiescent waking in the rest of the body.[98] The essential function of sleep may be its restorative effect on the brain: "Sleep is of the brain, by the brain and for the brain."[99] Furthermore, this includes almost any brain, no matter how small: sleep is observed to be a necessary behavior across most of the animal kingdom, including some of the least cognitively advanced animals, implying that sleep is essential to the most fundamental brain processes, i.e. neuronal firing. This shows that sleep is vital even when there is no need for other functions of sleep, such asmemory consolidation or dreaming.[6]
It has been widely accepted that sleep must support the formation of long-term memory, and generally increasing previous learning and experiences recalls. However, its benefit seems to depend on the phase of sleep and the type of memory.[100] For example, declarative and procedural memory-recall tasks applied over early and late nocturnal sleep, as well as wakefulness controlled conditions, have been shown that declarative memory improves more during early sleep (dominated by SWS) while procedural memory during late sleep (dominated by REM sleep) does so.[101][102]
With regard to declarative memory, the functional role of SWS has been associated with hippocampal replays of previously encoded neural patterns that seem to facilitate long-term memory consolidation.[101][102] This assumption is based on the active system consolidation hypothesis, which states that repeated reactivations of newly encoded information in the hippocampus during slow oscillations in NREM sleep mediate the stabilization and gradual integration of declarative memory with pre-existing knowledge networks on the cortical level.[103] It assumes the hippocampus might hold information only temporarily and in a fast-learning rate, whereas the neocortex is related to long-term storage and a slow-learning rate.[101][102][104][105][106] This dialogue between the hippocampus and neocortex occurs in parallel with hippocampalsharp-wave ripples andthalamo-cortical spindles, synchrony that drives the formation of the spindle-ripple event which seems to be a prerequisite for the formation of long-term memories.[102][104][106][107]
Reactivation of memory also occurs during wakefulness and its function is associated with serving to update the reactivated memory with newly encoded information, whereas reactivations during SWS are presented as crucial for memory stabilization.[102] Based on targeted memory reactivation (TMR) experiments that use associated memory cues to triggering memory traces during sleep, several studies have been reassuring the importance of nocturnal reactivations for the formation of persistent memories in neocortical networks, as well as highlighting the possibility of increasing people's memory performance at declarative recalls.[101][105][106][107][108]
Furthermore, nocturnal reactivation seems to share the same neural oscillatory patterns as reactivation during wakefulness, processes which might be coordinated bytheta activity.[109] During wakefulness, theta oscillations have been often related to successful performance in memory tasks, and cued memory reactivations during sleep have been showing that theta activity is significantly stronger in subsequent recognition of cued stimuli as compared to uncued ones, possibly indicating a strengthening of memory traces and lexical integration by cuing during sleep.[110] However, the beneficial effect of TMR for memory consolidation seems to occur only if the cued memories can be related to prior knowledge.[111]
Dreams often feel like waking life, yet with added surrealism.
During sleep, especially REM sleep, humans tend to experience dreams. These are elusive and mostly unpredictable first-person experiences which seem logical and realistic to the dreamer while they are in progress, despite their frequently bizarre, irrational, and/or surreal qualities that become apparent when assessed after waking. Dreams often seamlessly incorporate concepts, situations, people, and objects within a person's mind that would not normally go together. They can include apparent sensations of all types, especially vision and movement.[112]
Dreams tend to rapidly fade from memory after waking. Some people choose to keep adream journal, which they believe helps them build dream recall and facilitate the ability to experiencelucid dreams.
A lucid dream is a type of dream in which the dreamer becomes aware that they are dreaming while dreaming. In a preliminary study, dreamers were able to consciouslycommunicate with experimenters via eye movements or facial muscle signals, and were able to comprehend complex questions and use working memory.[113]
Counterintuitively,penile erections during sleep are not more frequent during sexual dreams than during other dreams.[115] Theparasympathetic nervous system experiences increased activity during REM sleep which may cause erection of the penis or clitoris. In males, 80% to 95% of REM sleep is normally accompanied by partial to full penile erection, while only about 12% of men's dreams contain sexual content.[116]
Insomnia is a general term for difficulty falling asleep and/or staying asleep. Insomnia is the most common sleep problem, with many adults reporting occasional insomnia, and 10–15% reporting a chronic condition.[117] Insomnia can have many different causes, includingpsychological 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. The sleep environment may be improved 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 exercisemay 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.[118]Nonbenzodiazepine sleeping medications such asAmbien,Imovane, andLunesta (also known as "Z-drugs"), while initially believed to be entirely better and safer than earlier generations ofsedatives—includingbenzodiazepinedrugs—are now known to be similar in more ways than thought.White noise appears to be a promising treatment forinsomnia.[119]
Sleep duration measures the length of sleep, whereas sleep quality includes factors such as speed in falling asleep and whether sleep is unbroken.[120][121] Adequate quality sleep is linked with better mood and the abilities to express and quickly processemotion.[122]
Low quality sleep has been linked with health conditions likecardiovascular disease,obesity, andmental illness. While poor sleep is common among those with cardiovascular disease, some research indicates that poor sleep can be a contributing cause. Short sleep duration of less than seven hours is correlated withcoronary heart disease and increased risk of death from coronary heart disease. Sleep duration greater than nine hours is also correlated with coronary heart disease, as well asstroke and cardiovascular events.[123][124][125][126]
In both children and adults, short sleep duration is associated with an increased risk of obesity, with various studies reporting an increased risk of 45–55%. Other aspects of sleep health have been associated with obesity, including daytime napping, sleep timing, the variability of sleep timing, and low sleep efficiency. However, sleep duration is the most-studied for its impact on obesity.[123]
Sleep problems have been frequently viewed as a symptom of mental illness rather than a causative factor. However, a growing body of evidence suggests that they are both a cause and a symptom of mental illness. Insomnia is a significant predictor ofmajor depressive disorder; ameta-analysis of 170,000 people showed that insomnia at the beginning of a study period indicated a more than the twofold increased risk for major depressive disorder. Some studies have also indicated correlation between insomnia andanxiety,post-traumatic stress disorder, andsuicide. Sleep disorders can increase the risk ofpsychosis and worsen the severity of psychotic episodes.[123]
Sleep research also displaysdifferences in race and class. Short sleep and poor sleep are observed more frequently in ethnic minorities than in whites in the US. African-Americans report experiencing short durations of sleep five times more often than whites, possibly as a result of social and environmental factors. A study done in the USA suggested that higher rates of sleep apnea (and poorer responses to treatment) are suffered by children in disadvantaged neighborhoods (which, in context, includes a disproportionate effect on children of African-American descent).[127]
Sleep hygiene
Sleep health can be improved through implementing goodsleep hygiene habits. Having good sleep hygiene can help to improve your physical and mental health by providing your body with the necessary rejuvenation only restful sleep can provide.[128] Some ways to improve sleep health include going to sleep at consistent times every night, avoiding any electronic devices such as televisions in the bedroom, getting adequate exercise throughout your day, and avoiding caffeine in the hours before going to sleep. Another way to greatly improve sleep hygiene is by creating a peaceful and relaxing sleep environment. Sleeping in a dark and clean room with things like a white noise maker can help facilitate restful sleep.[129] However,noise, with the exception ofwhite noise, may not be good for sleep.
Stimulants, which inhibit sleep, includecaffeine, anadenosine antagonist;amphetamine,methamphetamine,MDMA,empathogen-entactogens, and related drugs;cocaine, which can alter the circadian rhythm,[135][136] andmethylphenidate, which acts similarly; andeugeroic drugs likemodafinil andarmodafinil with poorly understood mechanisms. Consuming high amounts of the stimulant caffeine can result in interrupted sleep patterns and sometimes sleep deprivation. This vicious cycle can result in drowsiness which can then result in a higher consumption of caffeine in order to stay awake the next day. This cycle can lead to decreased cognitive function and an overall feeling of fatigue.[137]
Some drugs may alter sleep architecture without inhibiting or inducing sleep. Drugs that amplify or inhibit endocrine and immune system secretions associated with certain sleep stages have been shown to alter sleep architecture.[138][139] The growth hormone releasing hormone receptor agonistMK-677 has been shown to increase REM in older adults as well as stage IV sleep in younger adults by approximately 50%.[140]
Diet
Dietary and nutritional choices may affect sleep duration and quality. One 2016 review indicated that a high-carbohydrate diet promoted a shorter onset to sleep and a longer duration of sleep than a high-fat diet.[141] A 2012 investigation indicated that mixedmicronutrients andmacronutrients are needed to promote quality sleep.[142] A varied diet containing fresh fruits and vegetables, lowsaturated fat, andwhole grains may be optimal for individuals seeking to improve sleep quality.[141]Epidemiological studies indicate fewer insomnia symptoms and better sleep quality with aMediterranean diet.[143][144] Two studies have indicated a benefit oftart cherry juice for insomnia, or for increasing sleep efficiency as well as total sleep time.[143] High-qualityclinical trials on long-term dietary practices are needed to better define the influence of diet on sleep quality.[141]
Research suggests that sleep patterns vary significantly across cultures.[145][146][147] The most striking differences are observed between societies that have plentiful sources of artificial light and ones that do not. The primary difference appears to be that pre-light cultures have more broken-up sleep patterns. For example, people without artificial light 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.[145] Duringpre-industrialEurope,biphasic (bimodal) sleeping was considered the norm. Sleep onset was determined not by a set bedtime, but by whether there were things to do.[148]
The boundaries between sleeping and waking are blurred in these societies. 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.[145]
Some societies display a fragmented sleep pattern in which people sleep at all times of the day and night for shorter periods. In manynomadic orhunter-gatherer societies, people sleep on and off throughout the day or night depending on what is happening. Plentifulartificial 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. In general, people sleep in a more concentrated burst through the night, going to sleep much later, although this is not always the case.[145]
HistorianA. Roger Ekirch 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."[149][150] Ekirch 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.[150] Today in most societies people sleep during the night, but in very hot climates they may sleep during the day.[151] DuringRamadan, many Muslims sleep during the day rather than at night.[152]
In some societies, people sleep with at least one other person (sometimes many) or with animals. In other cultures, people rarely sleep with anyone except for an intimate partner. In almost all societies, sleeping partners are strongly regulated by social standards. For example, a person might only sleep with theimmediate family, theextended family, a spouse or romantic partner, children, children of a certain age, children of a 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.[145]
People sleep in a variety of locations. Some sleep directly on the ground; others on a skin or blanket; others sleep on platforms orbeds. 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.[145]
Sleep has been seen in culture as similar to death since antiquity;[153] inGreek mythology,Hypnos (the god of sleep) andThanatos (the god of death) were both said to be the children ofNyx (the goddess of night).[153]John Donne,Samuel Taylor Coleridge,Percy Bysshe Shelley, John Keats and other poets have all written poems about the relationship between sleep and death.[153] Shelley describes them as "both so passing, strange and wonderful!"[153] Keats similarly poses the question: "Can death be sleep, when life is but a dream".[154] Many people consider dying in one's sleep is the most peaceful way to die.[153] Phrases such as "big sleep" and "rest in peace" are often used in reference to death,[153] possibly in an effort to lessen its finality.[153] Sleep and dreaming have sometimes been seen as providing the potential for visionary experiences. In medieval Irish tradition, in order to become afilí, the poet was required to undergo a ritual called theimbas forosnai, in which they would enter a mantic, trancelike sleep.[155][156]
Many cultural stories have been told about people falling asleep for extended periods of time.[157][158] The earliest of these stories is the ancient Greek legend ofEpimenides of Knossos.[157][159][160][161] According to the biographerDiogenes Laërtius, Epimenides was a shepherd on the Greek island ofCrete.[157][162] One day, one of his sheep went missing and he went out to look for it, but became tired and fell asleep in a cave underMount Ida.[157][162] When he awoke, he continued searching for the sheep, but could not find it,[157][162] so he returned to his old farm, only to discover that it was now under new ownership.[157][162] He went to his hometown, but discovered that nobody there knew him.[157] Finally, he met his younger brother, who was now an old man,[157][162] and learned that he had been asleep in the cave for fifty-seven years.[157][162]
Of the thematic representations of sleep in art, physician and sleep researcher Meir Kryger wrote, "[Artists] have intense fascination with mythology, dreams, religious themes, the parallel between sleep and death, reward, abandonment of conscious control, healing, a depiction of innocence and serenity, and the erotic."[168]
^Bingham R, Terrence S, Siegel J, Dyken ME, Czeisler C (February 2007)."Waking Up To Sleep"(Several conference videos). The Science Network.Archived from the original on 24 July 2011. Retrieved25 January 2008.
^abKonnikova, Maria (11 January 2014)."Goodnight. Sleep Clean".The New York Times. Retrieved18 February 2014.She called it the glymphatic system, a nod to its dependence on glial cells
^Brown, pp. 1118–1119: "Compared with wakefulness, sleep reduces brain energy demands, as suggested by the 44% reduction in the cerebral metabolic rate (CMR) of glucose (791) and a 25% reduction in the CMR of O2 (774) during sleep."
^abcVan Cauter E, Spiegel K (1999). "Circadian and Sleep Control of Hormonal Secretions". In Zee PC, Turek FW (eds.).Regulation of Sleep and Circadian Rhythms. pp. 397–425.
^Parmeggiani PL, ed. (2011).Systemic Homeostasis and Poikilostasis in Sleep: Is REM Sleep a Physiological Paradox?. London: Imperial College Press. pp. 12–5.ISBN978-1-94916-572-2.
^Fuller PM, Gooley JJ, Saper CB (December 2006). "Neurobiology of the sleep-wake cycle: sleep architecture, circadian regulation, and regulatory feedback".Journal of Biological Rhythms.21 (6):482–93.doi:10.1177/0748730406294627.PMID17107938.S2CID36572447.
^Zee PC, Turek FW (1999). "Introduction to Sleep and Circadian Rhythms". In Zee PC, Turek FW (eds.).Regulation of Sleep and Circadian Rhythms. pp. 1–17.
^abcdefgDerk-Jan D, Edgar DM (1999). "Circadian and Homeostatic Control of Wakefulness and Sleep". In Zee PC, Turek FW (eds.).Regulation of Sleep and Circadian Rhythms. pp. 111–147.
^abCzeisler CA, Wright Jr KP (1999). "Influence of Light on Circadian Rhythmicity in Humans". In Zee PC, Turek FW (eds.).Regulation of Sleep and Circadian Rhythms. pp. 149–180.
^abZlomanczuk P, Schwartz WJ (1999). "Cellular and Molecular Mechanisms of Circadian Rhythms in Mammals". In Zee PC, Turek FW (eds.).Regulation of Sleep and Circadian Rhythms. pp. 309–342.
^abWehr TA (1999). "The Impact of Changes in Nightlength (Scotoperiod) on Human Sleep". In Zee PC, Turek FW (eds.).Regulation of Sleep and Circadian Rhythms. pp. 263–285.
^abWyatt JK, Ritz-De Cecco A, Czeisler CA, Dijk DJ (October 1999). "Circadian temperature and melatonin rhythms, sleep, and neurobehavioral function in humans living on a 20-h day".The American Journal of Physiology.277 (4 Pt 2): R1152-63.doi:10.1152/ajpregu.1999.277.4.r1152.PMID10516257.S2CID4474347.... 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
^Gottselig JM, Adam M, Rétey JV, Khatami R, Achermann P, Landolt HP (March 2006). "Random number generation during sleep deprivation: effects of caffeine on response maintenance and stereotypy".Journal of Sleep Research.15 (1):31–40.doi:10.1111/j.1365-2869.2006.00497.x.PMID16490000.S2CID10355305.
^Iglowstein I, Jenni OG, Molinari L, Largo RH (February 2003). "Sleep duration from infancy to adolescence: reference values and generational trends".Pediatrics.111 (2):302–7.doi:10.1542/peds.111.2.302.PMID12563055.S2CID8727836.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...
^Dagan Y (February 2002)."Circadian rhythm sleep disorders (CRSD)"(PDF).Sleep Medicine Reviews.6 (1):45–54.doi:10.1053/smrv.2001.0190.PMID12531141. Archived fromthe original(PDF: full text) on 27 February 2008. Retrieved5 June 2016.Early onset of CRSD, the ease of diagnosis, the high frequency of misdiagnosis and erroneous treatment, the potentially harmful psychological and adjustment consequences, and the availability of promising treatments, all indicate the importance of greater awareness of these disorders.
^Harvey AG, Payne S (March 2002). "The management of unwanted pre-sleep thoughts in insomnia: distraction with imagery versus general distraction".Behaviour Research and Therapy.40 (3):267–77.doi:10.1016/s0005-7967(01)00012-2.PMID11863237.S2CID16647017.
^Dijk DJ, Lockley SW (February 2002). "Integration of human sleep-wake regulation and circadian rhythmicity".Journal of Applied Physiology.92 (2):852–62.doi:10.1152/japplphysiol.00924.2001.PMID11796701.S2CID2502686.Consolidation of sleep for 8 h or more is only observed when sleep is initiated ~6–8 h before the temperature nadir.
^"How Much Sleep Do I Need?".CDC.gov. Centers for Disease Control and Prevention (CDC). 14 September 2022.Archived from the original on 2 November 2023.Last Reviewed: September 14, 2022. Source: National Center for Chronic Disease Prevention and Health Promotion, Division of Population Health.
^Schmid SM, Hallschmid M, Schultes B (January 2015). "The metabolic burden of sleep loss".The Lancet. Diabetes & Endocrinology.3 (1):52–62.doi:10.1016/S2213-8587(14)70012-9.PMID24731536.
^Dahl RE (2009). "The regulation of sleep and arousal: Development and psychopathology".Development and Psychopathology.8 (1):3–27.doi:10.1017/S0954579400006945.S2CID143514600.
^Spruyt K, Aitken RJ, So K, Charlton M, Adamson TM, Horne RS (May 2008). "Relationship between sleep/wake patterns, temperament and overall development in term infants over the first year of life".Early Human Development.84 (5):289–96.doi:10.1016/j.earlhumdev.2007.07.002.PMID17707119.
^Plihal W, Born J (July 1997). "Effects of early and late nocturnal sleep on declarative and procedural memory".Journal of Cognitive Neuroscience.9 (4):534–47.doi:10.1162/jocn.1997.9.4.534.PMID23968216.S2CID3300300.
^Schreiner T, Göldi M, Rasch B (November 2015). "Cueing vocabulary during sleep increases theta activity during later recognition testing".Psychophysiology.52 (11):1538–43.doi:10.1111/psyp.12505.PMID26235609.
^J. Alan Hobson, Edward F. Pace-Scott, & Robert Stickgold (2000), "Dreaming and the brain: Toward a cognitive neuroscience of conscious states",Behavioral and Brain Sciences 23.
^Buman MP, King AC (2010). "Exercise as a Treatment to Enhance Sleep".American Journal of Lifestyle Medicine.4 (6):500–514.doi:10.1177/1559827610375532.S2CID73314918.
^Turek FW, Czeisler CA (1999). "Role of Melatonin in the Regulation of Sleep". In Zee PC, Turek FW (eds.).Regulation of Sleep and Circadian Rhythms. pp. 181–195.
^Mamelak, M.; Escriu, J. M.; Stokan, O. (April 1977). "The effects of gamma-hydroxybutyrate on sleep".Biological Psychiatry.12 (2):273–288.ISSN0006-3223.PMID192353.
^Copinschi, G.; Leproult, R.; Van Onderbergen, A.; Caufriez, A.; Cole, K. Y.; Schilling, L. M.; Mendel, C. M.; De Lepeleire, I.; Bolognese, J. A.; Van Cauter, E. (October 1997). "Prolonged oral treatment with MK-677, a novel growth hormone secretagogue, improves sleep quality in man".Neuroendocrinology.66 (4):278–286.doi:10.1159/000127249.ISSN0028-3835.PMID9349662.
^Ekirch AR (2001). "Sleep we have lost: pre-industrial slumber in the British Isles".The American Historical Review.106 (2):343–86.doi:10.2307/2651611.JSTOR2651611.PMID18680884.
^Jones BJ (2008).Washington Irving: An American Original. New York: Arcade Books. pp. 177–178.ISBN978-1-55970-836-4.
^Churchland, Patricia Smith (1 January 2005).A neurophilosophical slant on consciousness research. Progress in Brain Research. Vol. 149. Elsevier. pp. 285–293.
.jpg)
.jpg)
.jpg)
.jpg)
.jpg)
