| Sleep paralysis | |
|---|---|
 | |
| The Nightmare by Henry Fuseli (1781) is thought to be a depiction of sleep paralysis perceived as a demonic visitation. | |
| Specialty | Sleep medicine |
| Symptoms | Awareness but inability to move during waking or falling asleep |
| Duration | Less than a couple of minutes |
| Risk factors | Narcolepsy, obstructive sleep apnea, alcohol use, sleep deprivation |
| Diagnostic method | Based on description |
| Differential diagnosis | Narcolepsy, atonic seizure, hypokalemic periodic paralysis, night terrors |
| Treatment | Reassurance, sleep hygiene, cognitive behavioral therapy, antidepressants |
| Frequency | 8–50% |
Sleep paralysis is when, during awakening or falling asleep, a person is aware but unable to move or speak. During an episode, one may hallucinate (hear, feel, or see things that are not there), which often result in fear. Episodes generally last less than a couple of minutes. It may occur as a single episode or be recurrent.
The condition may occur in those who are otherwise healthy, those with narcolepsy, or may run in families as a result of specific genetic changes. The condition can be triggered by sleep deprivation, psychological stress, or abnormal sleep cycles. The underlying mechanism is believed to involve a dysfunction in REM sleep. Diagnosis is based on a person's description. Other conditions that can present similarly include narcolepsy, atonic seizure, and hypokalemic periodic paralysis.
Treatment options for sleep paralysis have been poorly studied. It is recommended that people be reassured that the condition is common and generally not serious. Other efforts that may be tried include sleep hygiene, cognitive behavioral therapy, and antidepressants.
Between 8% and 50% of people experience sleep paralysis at some time. About 5% of people have regular episodes. Males and females are affected equally. Sleep paralysis has been described throughout history. It is believed to have played a role in the creation of stories about alien abduction and other paranormal events.
Signs and symptoms
The central symptom of sleep paralysis is being unable to move during awakening.
Imagined sounds such as humming, hissing, static, zapping and buzzing noises are reported during sleep paralysis. Other sounds such as voices, whispers and roars are also experienced. These symptoms are usually accompanied by intense emotions such as fear and panic. People also have sensations of being dragged out of bed or of flying, numbness, and feelings of electric tingles or vibrations running through their body.
Sleep paralysis may include hypnogogic hallucinations, such as a supernatural creature suffocating or terrifying the individual, accompanied by a feeling of pressure on one's chest and difficulty breathing. Another example of a hallucination involves a menacing shadowy figure entering one's room or lurking outside one's window, while the subject is paralyzed.
The content and interpretation of these hallucinations are driven by
fear, somatic sensations, REM-induced sexual arousal, and REM mentation
which are embedded in the sleeper's cultural narrative.
REM sleep physiology and somatic symptoms coupled with the
awareness that one is paralyzed can generate a variety of psychological
symptoms during sleep paralysis, including fear and worry that are aggravated by catastrophic cognitions about the attack. This can activate a fight-flight
reaction and panic-like arousal. Consequently, when the person attempts
to escape the paralysis, somatic symptoms and arousal are exacerbated,
as an execution of motor programs in the absence of dampening proprioceptive feedback can lead to heightened sensations of bodily tightness and pressure, and even pain and spasms in limbs.
Pathophysiology
The pathophysiology of sleep paralysis has not been concretely identified, although there are several theories about its cause. The first of these stems from the understanding that sleep paralysis is a parasomnia resulting from dysfunctional overlap of the REM and waking stages of sleep.
Polysomnographic studies found that individuals who experience sleep
paralysis have shorter REM sleep latencies than normal along with
shortened NREM and REM sleep cycles, and fragmentation of REM sleep.
This study supports the observation that disturbance of regular
sleeping patterns can instigate an episode of sleep paralysis, because
fragmentation of REM sleep commonly occurs when sleep patterns are
disrupted and has now been seen in combination with sleep paralysis.
Another major theory is that the neural functions that regulate
sleep are out of balance in such a way that causes different sleep
states to overlap. In this case, cholinergic sleep on neural populations are hyperactivated and the serotonergic sleep off neural populations are under-activated.
As a result, the cells capable of sending the signals that would allow
for complete arousal from the sleep state, the serotonergic neural
populations, have difficulty in overcoming the signals sent by the cells
that keep the brain in the sleep state.
During normal REM sleep, the threshold for a stimulus to cause arousal
is greatly elevated. Under normal conditions, medial and vestibular nuclei, cortical, thalamic, and cerebellar centers coordinate things such as head and eye movement, and orientation in space.
However, in individuals with SP, there is almost no blocking of
exogenous stimuli, which means it is much easier for a stimulus to
arouse the individual. There may also be a problem with the regulation of melatonin, which under normal circumstances regulates the serotonergic neural populations. Melatonin is typically at its lowest point during REM sleep.
Inhibition of melatonin at an inappropriate time would make it
impossible for the sleep off neural populations to depolarize when
presented with a stimulus that would normally lead to complete arousal. The vestibular nuclei in particular has been identified as being closely related to dreaming during the REM stage of sleep.
According to this hypothesis, vestibular-motor disorientation, unlike
hallucinations, arise from completely endogenous sources of stimuli.
This could explain why the REM and waking stages of sleep overlap
during sleep paralysis, and definitely explains the muscle paralysis
experienced on awakening.
If the effects of sleep on neural populations cannot be counteracted,
characteristics of REM sleep are retained upon awakening. Common
consequences of sleep paralysis include headaches, muscle pains or
weakness and/or paranoia. As the correlation with REM sleep suggests,
the paralysis is not complete: use of EOG
traces shows that eye movement is still possible during such episodes;
however, the individual experiencing sleep paralysis is unable to speak.
Research has found a genetic component in sleep paralysis.
The characteristic fragmentation of REM sleep, hypnopompic, and
hypnagogic hallucinations have a heritable component in other
parasomnias, which lends credence to the idea that sleep paralysis is
also genetic. Twin studies have shown that if one twin of a monozygotic pair (identical twins) experiences sleep paralysis that other twin is very likely to experience it as well.
The identification of a genetic component means that there is some sort
of disruption of a function at the physiological level. Further studies
must be conducted to determine whether there is a mistake in the
signaling pathway for arousal as suggested by the first theory
presented, or whether the regulation of melatonin or the neural
populations themselves have been disrupted.
Hallucinations
A picture of a succubus-like vision, in contrast to the incubus. My Dream, My Bad Dream, 1915, by Fritz Schwimbeck
Several types of hallucinations have been linked to sleep paralysis:
the belief that there is an intruder in the room, the presence of an incubus, and the sensation of floating.
A neurological hypothesis is that in sleep paralysis the mechanisms
which usually coordinate body movement and provide information on body
position become activated and, because there is no actual movement,
induce a floating sensation.
The intruder and Incubus
hallucinations highly correlate with one another, and moderately
correlated with the third hallucination, vestibular-motor
disorientation, also known as out-of-body experiences, which differ from the other two in not involving the threat-activated vigilance system.
Several theories have been proposed to explain the hallucinations
that may accompany sleep paralysis, but there is currently no research
that supports a neurological model.
Threat hyper-vigilance
A hyper-vigilant state created in the midbrain may further contribute to hallucinations.
More specifically, the emergency response is activated in the brain
when individuals wake up paralyzed and feel vulnerable to attack.
This helplessness can intensify the effects of the threat response well
above the level typical of normal dreams, which could explain why such
visions during sleep paralysis are so vivid.
The threat-activated vigilance system is a protective mechanism that
differentiates between dangerous situations and determines whether the
fear response is appropriate.
The hyper-vigilance response can lead to the creation of endogenous stimuli that contribute to the perceived threat.
A similar process may explain hallucinations, with slight variations,
in which an evil presence is perceived by the subject to be attempting
to suffocate them, either by pressing heavily on the chest or by
strangulation.
A neurological explanation holds that this results from a combination
of the threat vigilance activation system and the muscle paralysis
associated with sleep paralysis that removes voluntary control of
breathing. Several features of REM breathing patterns exacerbate the feeling of suffocation. These include shallow rapid breathing, hypercapnia, and slight blockage of the airway, which is a symptom prevalent in sleep apnea patients.
According to this account, the subjects attempt to breathe deeply
and find themselves unable to do so, creating a sensation of
resistance, which the threat-activated vigilance system interprets as an
unearthly being sitting on their chest, threatening suffocation.
The sensation of entrapment causes a feedback loop when the fear of
suffocation increases as a result of continued helplessness, causing the
subjects to struggle to end the SP episode.
Diagnosis
Sleep paralysis is mainly diagnosed via clinical interview and ruling out other potential sleep disorders that could account for the feelings of paralysis.
The main disorder that is checked for is narcolepsy due to the high
prevalence of narcolepsy in conjunction with sleep paralysis. The
availability of a genetic test for narcolepsy makes this an easy disorder to rule out. Several measures are available to reliably diagnose (e.g., the fearful isolated sleep paralysis interview) or screen (Munich Parasomnia Screening) for recurrent isolated sleep paralysis.
Classification
Episodes of sleep paralysis can occur in the context of several medical conditions (e.g., narcolepsy, hypokalemia). When episodes occur independent of these conditions or substance use, it is termed "isolated sleep paralysis" (ISP).
When ISP episodes are more frequent and cause clinically significant
distress and/or interference, it is classified as "recurrent isolated
sleep paralysis"(RISP). Episodes of sleep paralysis, regardless of
classification, are generally short (1–6 minutes), but longer episodes
have been documented.
With RISP the individual can also suffer back-to-back episodes of sleep
paralysis in the same night, which is unlikely in individuals who
suffer from ISP.
It can be difficult to differentiate between cataplexy brought on by narcolepsy and true sleep paralysis, because the two phenomena are physically indistinguishable.
The best way to differentiate between the two is to note when the
attacks occur most often. Narcolepsy attacks are more common when the
individual is falling asleep; ISP and RISP attacks are more common upon
awakening.
Prevention
Several
circumstances have been identified that are associated with an
increased risk of sleep paralysis. These include insomnia, sleep
deprivation, an erratic sleep schedule, stress, and physical fatigue. It
is also believed that there may be a genetic component in the
development of RISP, because there is a high concurrent incidence of
sleep paralysis in monozygotic twins. Sleeping in the supine position has been found an especially prominent instigator of sleep paralysis.
Sleeping in the supine position is believed to make the sleeper
more vulnerable to episodes of sleep paralysis because in this sleeping
position it is possible for the soft palate to collapse and obstruct the
airway. This is a possibility regardless of whether the individual has been diagnosed with sleep apnea
or not. There may also be a greater rate of microarousals while
sleeping in the supine position because there is a greater amount of
pressure being exerted on the lungs by gravity.
While many factors can increase the risk for ISP or RISP, they can be avoided with minor lifestyle changes. By maintaining a regular sleep schedule and observing good sleep hygiene,
one can reduce chances of sleep paralysis. It helps subjects to reduce
the intake of stimulants and stress in daily life by taking up a hobby
or seeing a trained psychologist who can suggest coping mechanisms for
stress. However, some cases of ISP and RISP involve a genetic
factor—which means some people may find sleep paralysis unavoidable.
Practicing meditation regularly might also be helpful in preventing
fragmented sleep, and thus the occurrence of sleep paralysis. Research has shown that long-term meditation practitioners spend more time in slow wave sleep, and as such regular meditation practice could reduce nocturnal arousal and thus possibly sleep paralysis.
Treatment
Medical
treatment starts with education about sleep stages and the inability to
move muscles during REM sleep. People should be evaluated for narcolepsy if symptoms persist. The safest treatment for sleep paralysis is for people to adopt healthier sleeping habits. However, in more serious cases tricyclic antidepressants or selective serotonin reuptake inhibitors (SSRIs) may be used.
Despite the fact that these treatments are prescribed there is
currently no drug that has been found to completely interrupt episodes
of sleep paralysis a majority of the time.
Medications
Though
no large trials have taken place which focus on the treatment of sleep
paralysis, several drugs have promise in case studies. Two trials of GHB for people with narcolepsy demonstrated reductions in sleep paralysis episodes.
Cognitive-behavior therapy
Some of the earliest work in treating sleep paralysis was done using a culturally sensitive cognitive-behavior therapy called CA-CBT. The work focuses on psycho-education and modifying catastrophic cognitions about the sleep paralysis attack. This approach has previously been used to treat sleep paralysis in Egypt, although clinical trials are lacking.
The first published psychosocial treatment for recurrent isolated
sleep paralysis was cognitive-behavior therapy for isolated sleep
paralysis (CBT-ISP). CBT-ISP is manualized, has an adherence manual for research purposes, and is intended to both prevent and disrupt ISP episodes.
It begins with self-monitoring of symptoms, cognitive restructuring of
maladaptive thoughts relevant to ISP (e.g., "the paralysis will be
permanent"), and psychoeducation about the nature of sleep paralysis.
Prevention techniques include ISP-specific sleep hygiene and the
preparatory use of various relaxation techniques (e.g. diaphragmatic
breathing, mindfulness, progressive muscle relaxation, meditation).
Episode disruption techniques
are first practiced in session and then applied during actual attacks.
No controlled trial of CBT-ISP has yet been conducted to prove its
effectiveness.
Meditation-relaxation therapy
Meditation-relaxation (MR) therapy is a published direct treatment for sleep paralysis.
The treatment was partly derived from the neuroscientific hypothesis
suggesting that attempting movement during sleep paralysis (e.g., due to
panic-like reactions) can lead to neurological distortions of one's
"body image", possibly triggering hallucinations of shadowy human-like
figures.
The therapy is based on four steps applied during sleep paralysis: (1)
reappraisal of the meaning of the attack (cognitive reappraisal); which
entails closing one's eyes, avoid panicking and re-appraising the
meaning of the attack as benign. (2) psychological and emotional
distancing (emotion regulation); the sleeper reminds him- or herself
that catastrophizing the event (i.e., fear and worry) will worsen and
possibly prolong it; (3) inward focused-attention meditation; focusing
attention inward on an emotionally salient positive object; 4) muscle
relaxation; relaxing one's muscles, avoid controlling breathing and
avoid attempting to move.There are preliminary case reports supporting
this treatment, although no randomized clinical trials yet to show its
effectiveness.
Epidemiology
Sleep paralysis is equally experienced in both males and females.
Lifetime prevalence rates derived from 35 aggregated studies indicate
that approximately 8% of the general population, 28% of students, and
32% of psychiatric patients experience at least one episode of sleep
paralysis at some point in their lives.
Rates of recurrent sleep paralysis are not as well known, but 15%-45%
of those with a lifetime history of sleep paralysis may meet diagnostic
criteria for Recurrent Isolated Sleep Paralysis.
In surveys from Canada, China, England, Japan and Nigeria, 20% to 60%
of individuals reported having experienced sleep paralysis at least once
in their lifetime.
In general, non-whites appear to experience sleep paralysis at higher
rates than whites, but the magnitude of the difference is rather small.
Approximately 36% of the general population that experiences isolated
sleep paralysis is likely to develop it between 25 and 44 years of age.
Isolated sleep paralysis is commonly seen in patients that have
been diagnosed with narcolepsy. Approximately 30–50% of people that have
been diagnosed with narcolepsy have experienced sleep paralysis as an
auxiliary symptom.
A majority of the individuals who have experienced sleep paralysis
have sporadic episodes that occur once a month to once a year. Only 3%
of individuals experiencing sleep paralysis that is not associated with a
neuromuscular disorder have nightly episodes.
Sleep paralysis could lead the individual to acquire conditioned
fear of the experience ("worry attacks"), resulting in more nighttime
awakening and fragmented sleep (because of nocturnal arousal and
hyper-alertness to symptoms of paralysis), making the person more likely
to have sleep paralysis in the future.
Society and culture
Etymology
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A 19th century version of Füssli's The Nightmare (1781)
The original definition of sleep paralysis was codified by Samuel Johnson in his A Dictionary of the English Language as nightmare, a term that evolved into our modern definition. The term was first used and dubbed by British neurologist, S.A.K. Wilson in his 1928 dissertation, The Narcolepsies. Such sleep paralysis was widely considered the work of demons, and more specifically incubi, which were thought to sit on the chests of sleepers. In Old English the name for these beings was mare or mære (from a proto-Germanic *marōn, cf. Old Norse mara), hence comes the mare in the word nightmare. The word might be cognate to Greek Marōn (in the Odyssey) and Sanskrit Māra.
Cultural significance and priming
Le Cauchemar (The Nightmare), by Eugène Thivier (1894)
Although
the core features of sleep paralysis (e.g., atonia, a clear sensorium,
and frequent hallucinations) appear to be universal, the ways in which
they are experienced vary according to time, place, and culture. Over 100 terms have been identified for these experiences. Some scientists have proposed sleep paralysis as an explanation for reports of paranormal phenomena such as ghosts, parasites, alien visits, demons or demonic possession, alien abduction experiences, the Night Hag and shadow people haunting.
The night hag is a generic name for a fantastical creature from
the folklore of various peoples which is used to explain the phenomenon
of sleep paralysis. A common description is that a person feels a
presence of a supernatural malevolent being which immobilizes the person
as if sitting on his/her chest.
Various cultures have various names for this phenomenon and/or
supernatural character. For example, sleep paralysis is referred to as a
Pandafeche attack in Italy.
Among Italians the Pandafeche may refer to an evil witch, sometimes a ghost-like-spirit or a terrifying cat-like creature. Sleep paralysis among Cambodians is known as, “the ghost pushes you down,” and entails the belief in dangerous visitations from deceased relatives. In Egypt, sleep paralysis is conceptualized as a terrifying Jinn attack. The Jinn (i.e., evil genies) may terrorize and even kill its victims. Sleep paralysis is sometimes interpreted as space alien abduction in the United States.
According to some scientists culture may be a major factor in shaping sleep paralysis.
When sleep paralysis is interpreted through a particular cultural
filter, it may take on greater salience. For example, if sleep paralysis
is feared in a certain culture, this fear could lead to conditioned
fear, and thus worsen the experience, in turn leading to higher rates.
Consistent with this idea, high rates and long durations of immobility
during sleep paralysis have been found in Egypt, where there are
elaborate beliefs about sleep paralysis, involving malevolent
spirit-like creatures, the Jinn.
Research has found that sleep paralysis is associated with great fear and fear of impending death in 50% of sufferers in Egypt.
A study comparing rates and characteristics of sleep paralysis in Egypt
and Denmark found that the phenomenon is three times more common in
Egypt versus Denmark.
In Denmark, unlike Egypt, there are no elaborate supernatural beliefs
about sleep paralysis, and the experience is often interpreted as an odd
physiological event, with overall shorter sleep paralysis episodes and
fewer people (17%) fearing that they could die from it.
Literature
Various forms of magic and spiritual possession were also advanced as causes in literature. In nineteenth century Europe, the vagaries of diet were thought to be responsible. For example, in Charles Dickens's A Christmas Carol, Ebenezer Scrooge attributes the ghost
he sees to "... an undigested bit of beef, a blot of mustard, a crumb
of cheese, a fragment of an underdone potato..." In a similar vein, the Household Cyclopedia (1881) offers the following advice about nightmares:
- Great attention is to be paid to regularity and choice of diet. Intemperance of every kind is hurtful, but nothing is more productive of this disease than drinking bad wine. Of eatables those which are most prejudicial are all fat and greasy meats and pastry... Moderate exercise contributes in a superior degree to promote the digestion of food and prevent flatulence; those, however, who are necessarily confined to a sedentary occupation, should particularly avoid applying themselves to study or bodily labor immediately after eating... Going to bed before the usual hour is a frequent cause of night-mare, as it either occasions the patient to sleep too long or to lie long awake in the night. Passing a whole night or part of a night without rest likewise gives birth to the disease, as it occasions the patient, on the succeeding night, to sleep too soundly. Indulging in sleep too late in the morning, is an almost certain method to bring on the paroxysm, and the more frequently it returns, the greater strength it acquires; the propensity to sleep at this time is almost irresistible.
J. M. Barrie, the author of the Peter Pan
stories, may have had sleep paralysis. He said of himself ‘In my early
boyhood it was a sheet that tried to choke me in the night.’
He also described several incidents in the Peter Pan stories that
indicate that he was familiar with an awareness of a loss of muscle tone
whilst in a dream-like state. For example, Maimie is asleep but calls
out ‘What was that....It is coming nearer! It is feeling your bed with
its horns-it is boring for [into] you’.
and when the Darling children were dreaming of flying, Barrie says
‘Nothing horrid was visible in the air, yet their progress had become
slow and labored, exactly as if they were pushing their way through
hostile forces. Sometimes they hung in the air until Peter had beaten on
it with his fists.’ Barrie describes many parasomnias and neurological symptoms in his books and uses them to explore the nature of consciousness from an experiential point of view.
Documentary films
The Nightmare
is a 2015 documentary that discusses the causes of sleep paralysis as
seen through extensive interviews with participants, and the experiences
are re-enacted by professional actors. In synopsis, it proposes that
such cultural memes as alien abduction, the near death experience and shadow people can, in many cases, be attributed to sleep paralysis. The "real-life" horror film debuted at the Sundance Film Festival on January 26, 2015 and premiered in theaters on June 5, 2015.
