With the exception of maybe your eyes and your breathing, you find yourself virtually frozen in bed. For some this situation is then made exponentially worse by hallucinations that manifest in their surroundings, threatening them while they cannot even muster a scream. Such experiences are caused by what’s known as sleep paralysis (or SP), a phenomenon that occurs every night in the typical sleeper to prevent him or her from acting out dreams during REM sleep. When this paralysis outlasts sleep, as it sometimes can, you get instances like those described above.
While sleep paralysis occurs in the typical sleeper virtually every night, the phrase itself has become synonymous with the experiences that can more precisely be referred to as awareness during sleep paralysis (ASP), or isolated sleep paralysis (iSP). Isolated Sleep Paralysis occurs alone without being part of another sleeping disorder.
The two major classifications of sleep paralysis are isolated sleep paralysis (ISP) and the significantly rarer recurrent isolated sleep paralysis (RISP). ISP episodes are infrequent, and may occur only once in an individual’s lifetime, while recurrent isolated sleep paralysis is a chronic condition, and can recur throughout a person’s lifetime. RISP episodes can last for up to an hour or longer, and have a much higher occurrence of perceived out of body experiences, while ISP episodes are generally short (usually no longer than one minute) and are typically associated with the intruder and incubus visitations. 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.
Sleep paralysis episodes can also more rarely be a sign of the sleep disorder narcolepsy, and in these cases it is no longer referred to as isolated. Narcolepsy itself is another fascinating sleep disorder, involving patients suffering attacks of REM sleep unexpectedly at various times during the day. Despite being associated sometimes with narcolepsy though, awareness during sleep paralysis happens quite frequently in even the most normal of sleepers. In fact, it is said that over 50% of people will experience at least one episode of ASP during their lifetime.
What causes sleep paralysis?
To answer this question it’s necessary to know a bit about REM (Rapid Eye Movement) sleep, the stage of sleep where most of your dreams occur. Every night during REM sleep the body shuts down virtually all muscle activity to prevent you from acting out those dreams. Your brain is extraordinarily active as you dream, as you might be able to imagine if you think about the complex environments and actions it is conjuring. The only thing that prevents your body from moving to carry out the scene your brain is living is a mechanism that kicks in to paralyze it.
This mechanism is referred to as REM atonia, or REM sleep paralysis. Very rarely, this mechanism can malfunction in some people with a condition that’s quite the opposite of ASP known as REM sleep behavior disorder (or RBD). People with RBD often thrash around (or even walk and sometimes even drive!) in a manner that corresponds with what’s happening in their dream world. In other words, they act out their dreams, because the paralysis that should be there during the dream is not.
On the other end of the spectrum, occasionally this REM atonia can work too well, in a sense, and trickle over into the waking state for a short period. This is exactly what is believed to happen in instances of awareness during sleep paralysis.
Beyond some logical connections we can make between SP and the dream world, the precise nature of SP apparitions remains one of the biggest mysteries in dream research. The visions experienced during sleep paralysis are known as hypnagogic hallucinations (HH), or more precisely, if they occur upon awakening, hypnopompic hallucinations. After reading the causes of sleep paralysis section above, we can understand how closely tied SP is to REM sleep. We also understand that REM sleep is the time of the night where our most vivid dreams occur. In episodes of ASP, our wakefulness intrudes on REM sleep in a way that doesn’t fully extinguish the REM (hence the paralyzation). Our eyes are often open but we are still immersed in ways in the dream state. The fact that we can perceive our surroundings enables us to project parts of a dream outward onto the actual physical space around us.
Additionally, the part of the brain that is most responsible for the intense emotions we feel, the amygdala, is already heightened from our dream state during sleep, possibly contributing to the ease at which our fears and terrors are projected during these hypnagogic hallucinations. These connections between the dream world and our waking state during ASP make a ton of sense intuitively, but of course there’s a lot more room for further verifiable explanation as to the exactness of how sleep paralysis hallucinations occur.
Many people who experience sleep paralysis are struck with a deep sense of terror when they sense a menacing presence in the room while paralyzed—hereafter referred to as the intruder. A neurological interpretation of this phenomenon is that it results from a hyper-vigilant state created in the midbrain. 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. Normally the threat-activated vigilance system (TAVS) is a protective mechanism to differentiate between dangerous situations and to determine whether the fear response is appropriate. Some hypothesize that the threat vigilance system is evolutionarily biased to interpret ambiguous stimuli as dangerous, because “erring on the side of caution” increases survival chances. This hypothesis could account for why the threatening presence is perceived as being evil. The amygdala is heavily involved in the threat activation response mechanism, which is implicated in both intruder and incubus SP visions.
The specific pathway through which the TAVS acts is not well understood. One possibility is that the thalamus receives sensory information and sends it on the amygdala, which regulates emotional experience. Another is that the amygdaloid complex, anterior cingulate, and the structures in the pontine tegmentum interact to create the vision. It is also highly possible that SP hallucinations could result from a combination of these. The anterior cingulate has an extensive array of cortical connections to other cortical areas, which enables it to integrate the various sensations and emotions into the unified sensorium we experience. The amygdaloid complex helps us interpret emotional experience and act appropriately. This is conducive to directing the individual’s attention to the most pertinent stimuli in a potentially dangerous situation so that the individual can take self-protective measures. At low levels of activation, the TAVS may only give rise to the vague feeling of a presence. At increased levels, auditory, visual and touchable cues may become incorporated and interpreted as the approach of an intruder.
The sensation choking may be nothing other than an awareness of the rapid and shallow breathing pattern characteristic of REM sleep, coupled with an SP-related inability to control one’s breathing pattern, that is to take a deep breath.
Hope this post clarifies your doubts on ghost infestation in your house! 😛
Please share your valuable thoughts and experiences, if any, in the comments section! 🙂
Various websites including wikipedia and webmd.