”. . . in the night time, when she was composing her self […] to sleep, sometimes she believed the devil lay upon her and held her down, sometimes that she was choaked […] by a great dog or thief lying upon her breast, so that she could hardly speak or breath, and when she endeavoured to throw off the burthen, she was not able to stir her members.”

This first report of what is today described as the parasomnia disorder “sleep paralysis”, was diagnosed already in 1664 as an “Incubus or the Night Mare[…]” by the Dutch doctor Isbrand van Diemerbroeck.
In the 1920s, the condition was then initially associated with cataplexy and narcolepsy before the term sleep paralysis was eventually coined around 1925. It is known today, that the classic symptoms are caused by rapid eye movement (REM) sleep dysregulation where the REM typical atony, muscle paralysis, is experienced during wakefulness. This state is usually accompanied by REM typical dream activity while the individual is consciously paralysed, but the ocular and respiratory systems are functioning, thus creating a sensation of dreaming with open eyes. The dream mentation during sleep paralysis is reported to be negatively valanced, vivid and multisensorial, which makes it a highly unpleasant and even terrifying condition for individuals affected. A common phenomenon is the hallucination of a bedroom intruder, mostly pictured as either a shadowy figure of humanoid appearance or other culture-specific, often super-natural beings (e.g. aliens), which are commonly perceived as to approach the individual. Hallucinations that often accompany the intruder’s threatening appearance are footsteps, noises and vocal sounds. As individuals have only limited control over respiration during episodes, they frequently experience subjective feelings of suffocation or general difficulty breathing, which is sometimes associated with the aforementioned hallucinated intruder. This hallucinatory phenomenon is called the incubus. A third category of hallucinations reported during sleep paralysis are vestibular-motor experiences leading to sensations of falling, flying or floating, that, in contrast to the intruder and incubus, are usually associated with positive feelings.
Sleep paralysis is a common symptom of the rare autoimmune sleep disorder narcolepsy, which is marked by excessive sleepiness at day, disrupted sleep at night, cataplexy (the sudden loss of muscle tone), as well as hypnogogic and hypnopompic hallucinations – so called “sleep hallucinations” occurring at the states leading into and out of sleep, respectively.
The majority of sleep paralyses occurs without relation to this sleep disorder though and is not of autoimmune nature. This condition is referred to as “isolated” sleep paralysis and has a much higher prevalence: a recent systematic review found that 7.6% of the general population reported at least one episode in their life course, whereas in students, lifetime rates of 28.3% were reached and in psychiatric patients even 31.9%. Episodes of isolated sleep paralysis are rather short-lasting with durations spanning a few seconds to 20 minutes with a mean duration of 6 minutes.

What causes Sleep Paralysis?

The Neurophysiological Bases

Poor sleep and sleep disruption, but also other forms of sleep dysregulations such as naps and jet lag have been associated with the presence of sleep paralysis, as they lead to a decoupling of perception and motor movement during REM sleep, causing the individual to wake up before the muscle paralysis has ended. REM sleep is initiated by inhibitory activation of so-called “REM-off cells” in reciprocity with “REM-on cells”, according to the activation synthesis theory. Mechanisms instantiated by the brainstem then work to inhibit motor output and simultaneously block sensory input, leading to muscle atony. Our dreams are thought to be generated as quasi-random activity organized into meaningful patterns by the frontal cortical areas. Especially at sleep onset REM periods, sleep paralysis has been found to be more pronounced. Sleeping in a supine position is also considered to be another typical risk factor.
Furthermore, REM neurophysiology has been associated with the three hallucinatory categories. Whereas the intruder and the incubus rely on threat and vigilance modulated systems involving the amygdala, the vestibular-motor experiences seem to be driven by the cerebellar and cortical vestibular centres as well as the brainstem.
Another symptomatic trait of sleep paralysis are out-of-body-experiences, where the individual may encounter body distortion and perceive the environment from a different perspective. It is hypothesized that the parietal lobes might be involved in this body distortion as the superior parietal lobule constructs neural body representations . An explanation that could account for hallucinated movements are messages sent from the motor cortex to the muscles, which are still paralyzed from the ongoing REM sleep.
Besides, a twin molecular study of sleep paralysis recently found a variation in the PER2 gene, which belongs to the inner clock genes, modulating our circadian rhythms and regulating our biological clock, that could be associated with sleep paralysis and thus indicates that the risk may partly be genetic.

Associated Psychopathologies and Personality Traits

Different psychopathologies are associated with sleep paralysis. Posttraumatic stress disorder (PTSD) and histories of trauma for instance have a higher prevalence. It is unclear, however, whether PTSD itself is the root cause of sleep paralysis in these patients or if the sleep disruptions and hypervigilance accompanying the disorder cause sleep paralysis. Other commonly associated disorders are generalized anxiety disorder, panic disorder, depression, social anxiety and death anxiety. Fear appears to be another bidirectional factor that has on the one hand been found to be predisposing of sleep paralysis and on the other hand to result out of episodes. Research suggests that not only the feeling of being paralysed, but also the co-occurring hallucinations reportedly evoke those feelings of fear. Similarly, there appears to be a bidirectional relationship between sleep paralysis and certain personality traits, including beliefs in the supernatural and high levels of imaginativeness and dissociation.

Treatment Options

Strangely, although the diagnosis of sleep paralysis has been around for such a long time already, not a single randomized controlled has been conducted for isolated sleep paralysis to date. Due to the lack of systematic research, results of the scarce empirical findings must be interpreted cautiously. Further, it is still unclear if isolated sleep paralysis and sleep paralysis accompanying narcolepsy respond to pharmacological treatments in a comparable manner.
Tricyclic antidepressants and selective serotonin reuptake inhibitors are the most popular treatment agents, as they are hypothesized to suppress REM sleep and henceforth also sleep paralysis. Another treatment option targets the underlying cause of sleep disruptions and promotes sleep hygiene techniques including regular sleeping times, abstinence of alcohol, caffeine or other stimulants before bed, as well as avoidance of a supine sleeping position.
Individuals are reported to often avoid involving their surroundings in fear of being afflicted with a serious mental illness or of not being taken seriously. Thus, oftentimes the distress of affected individuals can easily be reduced by simply educating sufferers that sleep paralysis is a recognized phenomenon which is terrifying, but harmless. Episodes might even be reduced afterwards through the potential effect of less pre-sleep anxiety and hence less dysregulated sleep.
When sleep paralysis is still inevitable though, a strategy to break out of an episode might be of help. As during REM sleep smaller muscles are not paralysed to the same extent as larger muscles, some people report that moving small extremities such as their tongue, toes or fingers through will power, helps ending an episode. If people manage to break through, getting up and moving around might prevent the immediate onset of a following episode. Of course, just as any dream or sleep state, sleep paralysis can also quickly be ended by the touch or voice of another person.

Where are we today?

Sleep paralysis is a long-known condition that, although harmless in nature, causes a lot of fear and suffering for affected individuals. However, the lack of controlled randomized studies makes it difficult to ascertain the neural and physiological bases and hence potential treatment options. Furthermore, varying definitions of sleep paralysis and different assessment methods add to a high degree of variability between studies. We hope that more research will be conducted in this field soon to provide relief for sufferers.
Until then: know your monsters and sleep well!