Have you ever awakened from a deep sleep and then found that you could not move or speak? You could see and sense things in the room but you could not move. This frightening condition is known as isolated sleep paralysis (ISP).

ISP is a relatively common experience typically characterized by an inability to move or speak after waking up as well by formation of a fixed conviction that someone or something is in the room with you or is actually on top of you! Typically the sensed presence of this other being is considered to be dangerous or malevolent. Thus the individual who has an ISP usually finds the experience quite frightening.

Up to 58% of people have an ISP at least once in their lives but some people have them much more often. ISPs are very common in people with narcolepsy for example. ISPs typically occur after an aborted REM episode (the sleeper begins to enter REM but then awakens) and are more common in people with histories of emotional trauma and people with psychiatric histories.

Nightmares and ISPs tend to occur together: people who report frequent nightmares tend also to report more frequent ISPs. In one recent study of some 90,000 healthy people in Japan it was found that males with sleep problems and nightmares had the highest frequencies of ISPs.

Sleep paralysis was known to the ancients and it was thought to constitute evidence for demonic supernatural forces. Apparently, the extraordinary conviction that some unseen but sensed supernatural presence had been near sleeper/dreamer during the experience convinced teh ancients that ISPs were evidence for spirit beings.

In 1664, the Dutch physician Isbrand Van Diemerbroeck (1609-1674) published a collection of cases of ISP and described some aspects of its phenomenology. That phenomenology was subjected to quantitative study most recently by Parker and Blackmore (2002).

In that study the authors used the Hall--Van de Castle dream content scoring system to quantitate the characteristics of ISPs that differentiate them from dreams. There were 4 times as many references to the body in ISP reports than in dream reports. In addition there were higher levels of physical aggression against the individual than that found in dreams. Unlike dreams which depict a variety of emotions, fear predominated in ISP reports.

Parker and Blackmore summarized the typical ISP as follows: "...takes place in a familiar, indoor setting (the bedroom). Often a presence or person not known to the dreamer is present. The presence is more often male (if gender is reported), but most common of all is that a sexless "creature" or "form" is sensed. Interactions with the presence are predominantly aggressive (although men do attempt to befriend it more often than women do). "Dreamers" often feel victimized by these interactions. They also report a much greater awareness of their body, particularly torso, which is accompanied by increased reports of negative emotion (fear). They often struggle to overcome the situation (paralysis) and meet with equal degree of success or failure (sometimes they are able to overcome the paralysis or not)" (Parker and Blackmore, 2002; p. 57)

Given this sort of experience it is not surprising that many investigators have suggested that so-called alien abduction experiences are really just elaborated ISPs. The sexless sensed presence is an alien who paralyzes the victim in order to do manipulations on the victim's body etc. Other investigators have suggested that ISPs might be one source of belief in supernatural or spirit beings.

But what really is the root mechanism of ISPs? Clearly ISPs are related to REM sleep as an ISP fundamentally is an aborted REM episode. The individual does not fully transition to the waking state. REM-related paralysis persists. But what is the source of the sensed presence? Why should we all hallucinate a sensed presence when transitioning from REM to waking?

One could tell a functional story as follows: Mother nature implanted within us an automatic predator-detector that gets triggered whenever we are in a drowsy or transitional state of consciousness as that is when we are most vulnerable to predation. The detection mechanism automatically ‘creates' a sensed presence that is malevolent because rapid response to a dangerous being is required when responding to threat.

Against this rather simple ‘just so' story is the core phenomenology of the experience itself. If an ISP reflects a trigger response mechanism designed to protect against predators/threats when recovering from transitional states of consciousness (like sleep) why is it that we do not feel aggressive when in an ISP? Why are we instead the victims of an aggression?

Yes, our minds construct an explanation for the paralysis but then that explanation militates against the predator-preparation explanation. If an ISP is just a confabulation for the paralysis it cannot be a defense mechanism. But against the confabulation account is the fact that we all confabulate the same story! Some supernaturally malevolent being has paralyzed us and is victimizing us via manipulation of our bodies.

Hufford DJ. (2005). Sleep paralysis as spiritual experience. Transcult Psychiatry. 2005 Mar;42(1):11-45.

Sharpless BA, Barber JP. (2011). Lifetime prevalence rates of sleep paralysis: a systematic review. Sleep Med Rev. Oct;15(5):311-5. Epub 2011 May 14.

Munezawa T, Kaneita Y, Osaki Y, Kanda H, Ohtsu T, Suzuki H, Minowa M, Suzuki K, Higuchi S, Mori J, Ohida T. (2011). Nightmare and sleep paralysis among Japanese adolescents: a nationwide representative survey. Sleep Med. Jan;12(1):56-64.

Kompanje EJ. 'The devil lay upon her and held her down'. (2008). Hypnagogic hallucinations and sleep paralysis described by the Dutch physician Isbrand van Diemerbroeck (1609-1674) in 1664.J Sleep Res. Dec;17(4):464-7. Epub 2008 Aug 5.

Parker, J.D. and Blackmore, S. J. (2002). Comparing the content of sleep paralysis and dream reports. Dreaming, V. 12, #1, 45-59.

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