Rapid eye movement sleep

Polysomnographic record of REM Sleep. EEG highlighted by red box. Eye movement highlighted by red line.

REM sleep is "paradoxical" because of its similarities to wakefulness. Although the body is paralyzed, the brain acts somewhat awake, with cerebral neurons firing with the same overall intensity as in wakefulness.[4][5] Electroencephalography during REM deep sleep reveals fast, low amplitude, desynchronized neural oscillation (brainwaves) that resemble the pattern seen during wakefulness which differ from the slow δ (delta) waves pattern of NREM deep sleep.[1][6] An important element of this contrast is the 3-10 Hz theta rhythm in the hippocampus[7] and 40–60 Hz gamma waves in the cortex; patterns of EEG activity similar to these rhythms are also observed during wakefulness.[8] The cortical and thalamic neurons in the waking and REM sleeping brain are more depolarized (fire more readily) than in the NREM deep sleeping brain.[9] Human theta wave activity predominates during REM sleep in both the hippocampus and the cortex.[10][11]

During REM sleep, electrical connectivity among different parts of the brain manifests differently than during wakefulness. Frontal and posterior areas are less coherent in most frequencies, a fact which has been cited in relation to the chaotic experience of dreaming. However, the posterior areas are more coherent with each other; as are the right and left hemispheres of the brain, especially during lucid dreams.[12][13]

Brain energy use in REM sleep, as measured by oxygen and glucose metabolism, equals or exceeds energy use in waking. The rate in non-REM sleep is 11–40% lower.[14]

Neural activity during REM sleep seems to originate in the brain stem, especially the pontine tegmentum and locus coeruleus. REM sleep is punctuated and immediately preceded by PGO (ponto-geniculo-occipital) waves, bursts of electrical activity originating in the brain stem.[15] (PGO waves have long been measured directly in cats but not in humans because of constraints on experimentation; however, comparable effects have been observed in humans during "phasic" events which occur during REM sleep, and the existence of similar PGO waves is thus inferred.)[13] These waves occur in clusters about every 6 seconds for 1–2 minutes during the transition from deep to paradoxical sleep.[6] They exhibit their highest amplitude upon moving into the visual cortex and are a cause of the "rapid eye movements" in paradoxical sleep.[16][17][14] Other muscles may also contract under the influence of these waves.[18]

Research in the 1990s using positron emission tomography (PET) confirmed the role of the brain stem and suggested that, within the forebrain, the limbic and paralimbic systems showed more activation than other areas.[4] The areas activated during REM sleep are approximately inverse to those activated during non-REM sleep[14] and display greater activity than in quiet waking. The "anterior paralimbic REM activation area" (APRA) includes areas linked with emotion, memory, fear and sex, and may thus relate to the experience of dreaming during REMS.[13][19] More recent PET research has indicated that the distribution of brain activity during REM sleep varies in correspondence with the type of activity seen in the prior period of wakefulness.[4]

The superior frontal gyrus, medial frontal areas, intraparietal sulcus, and superior parietal cortex, areas involved in sophisticated mental activity, show equal activity in REM sleep as in wakefulness. The amygdala is also active during REM sleep and may participate in generating the PGO waves, and experimental suppression of the amygdala results in less REM sleep.[20] The amygdala may also regulate cardiac function in lieu of the less active insular cortex.[4]

Compared to slow-wave sleep, both waking and paradoxical sleep involve higher use of the neurotransmitter acetylcholine, which may cause the faster brainwaves. The monoamine neurotransmitters norepinephrine, serotonin and histamine are completely unavailable. Injections of acetylcholinesterase inhibitor, which effectively increases available acetylcholine, have been found to induce paradoxical sleep in humans and other animals already in slow-wave sleep. Carbachol, which mimics the effect of acetylcholine on neurons, has a similar influence. In waking humans, the same injections produce paradoxical sleep only if the monoamine neurotransmitters have already been depleted.[1][21][22][23][24]

Two other neurotransmitters, orexin and gamma-Aminobutyric acid (GABA), seem to promote wakefulness, diminish during deep sleep, and inhibit paradoxical sleep.[1][25]

Unlike the abrupt transitions in electrical patterns, the chemical changes in the brain show continuous periodic oscillation.[26]

According to the activation-synthesis hypothesis proposed by Robert McCarley and Allan Hobson in 1975–1977, control over REM sleep involves pathways of "REM-on" and "REM-off" neurons in the brain stem. REM-on neurons are primarily cholinergic (i.e., involve acetylcholine); REM-off neurons activate serotonin and noradrenaline, which among other functions suppress the REM-on neurons. McCarley and Hobson suggested that the REM-on neurons actually stimulate REM-off neurons, thereby serving as the mechanism for the cycling between REM and non-REM sleep.[1][21][23][27] They used Lotka–Volterra equations to describe this cyclical inverse relationship.[28] Kayuza Sakai and Michel Jouvet advanced a similar model in 1981.[25] Whereas acetylcholine manifests in the cortex equally during wakefulness and REM, it appears in higher concentrations in the brain stem during REM.[29] The withdrawal of orexin and GABA may cause the absence of the other excitatory neurotransmitters;[30] researchers in recent years increasingly include GABA regulation in their models.[31]

Most of the eye movements in "rapid eye movement" sleep are in fact less rapid than those normally exhibited by waking humans. They are also shorter in duration and more likely to loop back to their starting point. About seven such loops take place over one minute of REM sleep. In slow-wave sleep the eyes can drift apart; however, the eyes of the paradoxical sleeper move in tandem.[32] These eye movements follow the ponto-geniculo-occipital waves originating in the brain stem.[16][17] The eye movements themselves may relate to the sense of vision experienced in the dream,[33] but a direct relationship remains to be clearly established. Congenitally blind people, who do not typically have visual imagery in their dreams, still move their eyes in REM sleep.[14] An alternative explanation suggests that the functional purpose of REM sleep is for procedural memory processing, and the rapid eye movement is only a side effect of the brain processing the eye-related procedural memory.[34][35]

Generally speaking, the body suspends homeostasis during paradoxical sleep. Heart rate, cardiac pressure, cardiac output, arterial pressure, and breathing rate quickly become irregular when the body moves into REM sleep.[36] In general, respiratory reflexes such as response to hypoxia diminish. Overall, the brain exerts less control over breathing; electrical stimulation of respiration-linked brain areas does not influence the lungs, as it does during non-REM sleep and in waking.[37] The fluctuations of heart rate and arterial pressure tend to coincide with PGO waves and rapid eye movements, twitches, or sudden changes in breathing.[38]

Erections of the penis (nocturnal penile tumescence or NPT) normally accompany REM sleep in rats and humans.[39] If a male has erectile dysfunction (ED) while awake, but has NPT episodes during REM, it would suggest that the ED is from a psychological rather than a physiological cause. In females, erection of the clitoris (nocturnal clitoral tumescence or NCT) causes enlargement, with accompanying vaginal blood flow and transudation (i.e. lubrication). During a normal night of sleep the penis and clitoris may be erect for a total time of from one hour to as long as three and a half hours during REM.[40]

Body temperature is not well regulated during REM sleep, and thus organisms become more sensitive to temperatures outside their thermoneutral zone. Cats and other small furry mammals will shiver and breathe faster to regulate temperature during NREMS but not during REMS.[41] With the loss of muscle tone, animals lose the ability to regulate temperature through body movement. (However, even cats with pontine lesions preventing muscle atonia during REM did not regulate their temperature by shivering.)[42] Neurons which typically activate in response to cold temperatures—triggers for neural thermoregulation—simply do not fire during REM sleep, as they do in NREM sleep and waking.[43]

Consequently, hot or cold environmental temperatures can reduce the proportion of REM sleep, as well as amount of total sleep.[44][45] In other words, if at the end of a phase of deep sleep, the organism's thermal indicators fall outside of a certain range, it will not enter paradoxical sleep lest deregulation allow temperature to drift further from the desirable value.[46] This mechanism can be 'fooled' by artificially warming the brain.[47]

REM atonia, an almost complete paralysis of the body, is accomplished through the inhibition of motor neurons. When the body shifts into REM sleep, motor neurons throughout the body undergo a process called hyperpolarization: their already-negative membrane potential decreases by another 2–10 millivolts, thereby raising the threshold which a stimulus must overcome to excite them. Muscle inhibition may result from unavailability of monoamine neurotransmitters (restraining the abundance of acetylcholine in the brainstem) and perhaps from mechanisms used in waking muscle inhibition.[48] The medulla oblongata, located between pons and spine, seems to have the capacity for organism-wide muscle inhibition.[2] Some localized twitching and reflexes can still occur.[49] Pupils contract.[18]

Lack of REM atonia causes REM behavior disorder, sufferers of which physically act out their dreams,[50] or conversely "dream out their acts", under an alternative theory on the relationship between muscle impulses during REM and associated mental imagery (which would also apply to people without the condition, except that commands to their muscles are suppressed).[51] This is different from conventional sleepwalking, which takes place during slow-wave sleep, not REM.[52] Narcolepsy by contrast seems to involve excessive and unwanted REM atonia—i.e., cataplexy and excessive daytime sleepiness while awake, hypnagogic hallucinations before entering slow-wave sleep, or sleep paralysis while waking.[53] Other psychiatric disorders including depression have been linked to disproportionate REM sleep.[54] Patients with suspected sleep disorders are typically evaluated by polysomnogram.[55][56]

Lesions of the pons to prevent atonia have induced functional "REM behavior disorder" in animals.[57]

Rapid eye movement sleep (REM) has since its discovery been closely associated with dreaming. Waking up sleepers during a REM phase is a common experimental method for obtaining dream reports; 80% of neurotypical people can give some kind of dream report under these circumstances.[58][59] Sleepers awakened from REM tend to give longer, more narrative descriptions of the dreams they were experiencing, and to estimate the duration of their dreams as longer.[14][60] Lucid dreams are reported far more often in REM sleep.[61] (In fact these could be considered a hybrid state combining essential elements of REM sleep and waking consciousness.)[14] The mental events which occur during REM most commonly have dream hallmarks including narrative structure, convincingness (experiential resemblance to waking life), and incorporation of instinctual themes.[14] Sometimes they include elements of the dreamer's recent experience taken directly from episodic memory.[4] By one estimate, 80% of dreams occur during REM.[62]

Hobson and McCarley proposed that the PGO waves characteristic of "phasic" REM might supply the visual cortex and forebrain with electrical excitement which amplifies the hallucinatory aspects of dreaming.[22][27] However, people woken up during sleep do not report significantly more bizarre dreams during phasic REMS, compared to tonic REMS.[60] Another possible relationship between the two phenomena could be that the higher threshold for sensory interruption during REM sleep allows the brain to travel further along unrealistic and peculiar trains of thought.[60]

Some dreaming can take place during non-REM sleep. "Light sleepers" can experience dreaming during stage 2 non-REM sleep, whereas "deep sleepers", upon awakening in the same stage, are more likely to report "thinking" but not "dreaming". Certain scientific efforts to assess the uniquely bizarre nature of dreams experienced while asleep were forced to conclude that waking thought could be just as bizarre, especially in conditions of sensory deprivation.[60][63] Because of non-REM dreaming, some sleep researchers have strenuously contested the importance of connecting dreaming to the REM sleep phase. The prospect that well-known neurological aspects of REM do not themselves cause dreaming suggests the need to re-examine the neurobiology of dreaming per se.[64] Some researchers (Dement, Hobson, Jouvet, for example) tend to resist the idea of disconnecting dreaming from REM sleep.[14][65]

Previous research has shown that SSRIs have an important effect on REM sleep neurobiology and dreaming, and serve to intensify dreaming in humans.[66] A study at Harvard Medical School in 2000 tested the effects of paroxetine and fluvoxamine on healthy young adult male and females for 31 days: a drug-free baseline week, 19 days on either paroxetine or fluvoxamine with morning and evening doses, and 5 days of absolute discontinuation.[67] Results showed that SSRI treatment decreased the average amount of dream recall frequency in comparison to baseline measurements as a result of serotonergic REM suppression.[67] Fluvoxamine increased the length of dream reporting, bizarreness of dreams as well as the intensity of REM sleep. These effects were the greatest during acute discontinuation compared to treatment and baseline days.[67] However, the subjective intensity of dreaming increased[67] and the proclivity to enter REM sleep was decreased during SSRI treatment compared to baseline and discontinuation days.[67]

After waking from REM sleep, the mind seems "hyperassociative"—more receptive to semantic priming effects. People awakened from REM have performed better on tasks like anagrams and creative problem solving.[68]

Sleep aids the process by which creativity forms associative elements into new combinations that are useful or meet some requirement.[69] This occurs in REM sleep rather than in NREM sleep.[70][71] Rather than being due to memory processes, this has been attributed to changes during REM sleep in cholinergic and noradrenergic neuromodulation.[70] High levels of acetylcholine in the hippocampus suppress feedback from hippocampus to the neocortex, while lower levels of acetylcholine and norepinephrine in the neocortex encourage the uncontrolled spread of associational activity within neocortical areas.[72] This is in contrast to waking consciousness, where higher levels of norepinephrine and acetylcholine inhibit recurrent connections in the neocortex. REM sleep through this process adds creativity by allowing "neocortical structures to reorganise associative hierarchies, in which information from the hippocampus would be reinterpreted in relation to previous semantic representations or nodes."[70]

Sleep in general aids memory. REM sleep may favor the preservation of certain types of memories: specifically, procedural memory, spatial memory, and emotional memory. In rats, REM sleep increases following intensive learning, especially several hours after, and sometimes for multiple nights. Experimental REM sleep deprivation has sometimes inhibited memory consolidation, especially regarding complex processes (e.g., how to escape from an elaborate maze).[97] In humans, the best evidence for REM's improvement of memory pertains to learning of procedures—new ways of moving the body (such as trampoline jumping), and new techniques of problem solving. REM deprivation seemed to impair declarative (i.e., factual) memory only in more complex cases, such as memories of longer stories.[98] REM sleep apparently counteracts attempts to suppress certain thoughts.[68]

According to the dual-process hypothesis of sleep and memory, the two major phases of sleep correspond to different types of memory. "Night half" studies have tested this hypothesis with memory tasks either begun before sleep and assessed in the middle of the night, or begun in the middle of the night and assessed in the morning.[99] Slow-wave sleep, part of non-REM sleep, appears to be important for declarative memory. Artificial enhancement of the non-REM sleep improves the next-day recall of memorized pairs of words.[100] Tucker et al. demonstrated that a daytime nap containing solely non-REM sleep enhances declarative memory but not procedural memory.[101] According to the sequential hypothesis the two types of sleep work together to consolidate memory.[102]

Sleep researcher Jerome Siegel has observed that extreme REM deprivation does not significantly interfere with memory. One case study of an individual who had little or no REM sleep due to a shrapnel injury to the brainstem did not find the individual's memory to be impaired. Antidepressants, which suppress REM sleep, show no evidence of impairing memory and may improve it.[87]

Graeme Mitchison and Francis Crick proposed in 1983 that by virtue of its inherent spontaneous activity, the function of REM sleep "is to remove certain undesirable modes of interaction in networks of cells in the cerebral cortex", which process they characterize as "unlearning". As a result, those memories which are relevant (whose underlying neuronal substrate is strong enough to withstand such spontaneous, chaotic activation) are further strengthened, whilst weaker, transient, "noise" memory traces disintegrate.[103] Memory consolidation during paradoxical sleep is specifically correlated with the periods of rapid eye movement, which do not occur continuously. One explanation for this correlation is that the PGO electrical waves, which precede the eye movements, also influence memory.[16] REM sleep could provide a unique opportunity for "unlearning" to occur in the basic neural networks involved in homeostasis, which are protected from this "synaptic downscaling" effect during deep sleep.[104]

REM sleep prevails most after birth, and diminishes with age. According to the "ontogenetic hypothesis", REM (also known in neonates as active sleep) aids the developing brain by providing the neural stimulation that newborns need to form mature neural connections.[105] Sleep deprivation studies have shown that deprivation early in life can result in behavioral problems, permanent sleep disruption, and decreased brain mass.[106][78] The strongest evidence for the ontogenetic hypothesis comes from experiments on REM deprivation and the development of the visual system in the lateral geniculate nucleus and primary visual cortex.[78]

Ioannis Tsoukalas of Stockholm University has hypothesized that REM sleep is an evolutionary transformation of a well-known defensive mechanism, the tonic immobility reflex. This reflex, also known as animal hypnosis or death feigning, functions as the last line of defense against an attacking predator and consists of the total immobilization of the animal so that it appears dead. Tsoukalas argues that the neurophysiology and phenomenology of this reaction shows striking similarities to REM sleep; for example, both reactions exhibit brainstem control, paralysis, hypocampal theta rhythm, and thermoregulatory changes.[107][108]

According to "scanning hypothesis", the directional properties of REM sleep are related to a shift of gaze in dream imagery. Against this hypothesis is that such eye movements occur in those born blind and in fetuses in spite of lack of vision. Also, binocular REMs are non-conjugated (i.e., the two eyes do not point in the same direction at a time) and so lack a fixation point. In support of this theory, research finds that in goal-oriented dreams, eye gaze is directed towards the dream action, determined from correlations in the eye and body movements of REM sleep behavior disorder patients who enact their dreams.[109]

Dr. David M. Maurice (1922-2002), an eye specialist and semi-retired adjunct professor at Columbia University, proposed that REM sleep was associated with oxygen supply to the cornea, and that aqueous humor, the liquid between cornea and iris, was stagnant if not stirred.[110] Among the supportive evidences, he calculated that if aqueous humor was stagnant, oxygen from iris had to reach cornea by diffusion through aqueous humor, which was not sufficient. According to the theory, when the animal is awake, eye movement (or cool environmental temperature) enables the aqueous humor to circulate. When the animal is sleeping, REM provides the much-needed stir to aqueous humor. This theory is consistent with the observation that fetuses, as well as eye-sealed newborn animals, spend much time in REM sleep, and that during a normal sleep, a person's REM sleep episodes become progressively longer deeper into the night. However, owls have REM sleep, but do not move their head more than in non-REM sleep[111] and is well known that owls' eyes are nearly immobile.[112]

Another theory suggests that monoamine shutdown is required so that the monoamine receptors in the brain can recover to regain full sensitivity.

The sentinel hypothesis of REM sleep was put forward by Frederick Snyder in 1966. It is based upon the observation that REM sleep in several mammals (the rat, the hedgehog, the rabbit, and the rhesus monkey) is followed by a brief awakening. This does not occur for either cats or humans, although humans are more likely to wake from REM sleep than from NREM sleep. Snyder hypothesized that REM sleep activates an animal periodically, to scan the environment for possible predators. This hypothesis does not explain the muscle paralysis of REM sleep; however, a logical analysis might suggest that the muscle paralysis exists to prevent the animal from fully waking up unnecessarily, and allowing it to return easily to deeper sleep.[113][114][115]

Jim Horne, a sleep researcher at Loughborough University, has suggested that REM in modern humans compensates for the reduced need for wakeful food foraging.[8]

Other theories are that REM sleep warms the brain, stimulates and stabilizes the neural circuits that have not been activated during waking, or creates internal stimulation to aid development of the CNS; while some argue that REM lacks any purpose, and simply results from random brain activation.[109][116]

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