By Sophya Yeoh
The thought of suddenly being trapped and unable to move is terrifying, especially when one is in the safest possible place they can be – their own bed. That is the reality for those suffering from sleep paralysis (SP) – the inability to move or speak when one is just falling asleep or waking up, however being able to open their eyes and report on their surroundings upon waking (John Fakoya et.al., 2018; Cheyne, 2005). Stories of those who have experienced it have circulated online, terrifying readers with claims of not being able to speak or move, feelings of helplessness, being overcome with trepidation. SP is often accompanied by the imagined appearance of a sinister “shadow creature” which may either be quite passive or aggressive towards the dreamer (Solomonova, 2018). Episodes normally conclude when the dreamer suddenly regains control of a part of the body, rolls off the bed, or when someone enters the room; but the individual is left with the mental impact of such an encounter (John Fakoya et. al., 2018).
Mentions of SP can be dated back to ancient civilisations, with the first known description coming from Hippocrates sometime around 400BC and the Greeks naming it ephialtes, loosely translating to “pounce upon someone” (de Sá & Mota-Rolim, 2016). During the Christian era of the Middle Ages, the Latin terms “incubus” (a male demon that engages in sexual activity with sleeping women) and “succubus” (its female counterpart) emerged, where they would sexually harass their victims in the night. Multiple cultures have very consistently described SP, although its phenomenology and spiritual interpretation varies. Solomonova (2018) even theorised that establishing SP in a particular belief may either allow the dreamer to recognise that SP is transient, or drive negative values of SP by providing reason for a supernatural assault.
SP episodes have been organised into three non-distinct groups based on REM processes – intruder, incubus, and unusual bodily sensations (Cheyne, 2003,2005). An intruder episode is distinguished by the dreamer sensing something or someone in their house or room; this may also be accompanied by seeing and/or hearing them move around. It can be noted that those who have experienced intruder SP attacks state that this entity often watches them, and they feel as though they are in the presence of evil (Solomonova, 2018). The incubus involves the sleeper also feeling a presence, however it is interpreted as a supernatural attack and coincides with breathing difficulties, pressure on the chest, and/or pain (Cheyne, 2003; Solomonova, 2018). Some of the most traumatic and aggressive incubus SP attacks recorded have involved the sleeper being sexually assaulted, or being abducted by aliens (McNally and Clancy, 2005). The third category, unusual bodily sensations, is vastly different as compared to the other two categories in which it has positive connotations, and may feature a presence, it is non-threatening (Solomona, 2018). Some individuals report a positive spiritual event and being visited by deceased family members, with researchers postulating that such an episode may aid with feelings of bereavement and grief (Belicki, Gulko, Ruzycki and Aristotle, 2003). Others mention having out of body experiences, in which they are the presence that watches their sleeping form, and have the ability to float and undergo feelings of bliss (Cheyne, 2005).
SP is neurophysiologically categorised as a rapid-eye-movement (REM) parasomnia (Cheyne, 2005; Solomonova, 2018). McNally and Clancy (2005) suggest that it is “the manifestation of discordance between the cognitive/perceptual and motor aspects of REM sleep”. REM sleep occurs in cycles with different stages of non-REM (NREM) when one falls asleep, with the cycles increasing in length as the night progresses (Altevogt and Colte, 2006). Dreams, nightmares, and lucid dreams mostly occur during REM sleep and is characterised by skeletal muscle atonia, increased airway resistance, reduced respiration rate, increased sensorimotor brain activity, sexual arousal, and more (Altevogt and Colte, 2006). These are related to the phenomena observed during SP with the inability to move, difficulty breathing, vivid hallucinations, and erotic factors respectively. The suppression of muscle tone is caused by the pons and the ventromedial medulla via motor neuron inhibition in the spinal cord, which happens to prevent the dreamer from acting out their actions (John Fakoya et.al., 2018).
The parietal lobe functions to process somatosensory, auditory and visual information to determine perception, self, and spatial recognition. It is likely to play a part in intruder and incubus episodes due to its disruption, with Jalal and Ramachandran (2014) proposing that the presence is the hallucinated projection of the genetically in-grained body image (homunculus) located in the right parietal lobe – the same neuronal circuits determining aesthetic and sexual preference of body morphology (including visual and limbic structures). This would therefore elucidate why the observed shadow figure is often humanoid.
The amygdala is a brain region that is thought to be the main structure for processing fear, eliciting physical responses to environmental stimuli via its connection to the hypothalamus and the autonomic nervous system. Within the amygdala is the Threat Activated Vigilance System (TAVS) which lowers sensory thresholds and biases perception while monitoring the environment for further indication of threat existence (Cheyne, 2003). Additionally, the TAVS bias is likely to perceive ambiguous stimuli as a threat and can sometimes be endogenously activated during REM sleep. This association with dream imagery will be biased similarly to the interpretation of environmental threat, resulting in maleficent forms manifesting in dreams and even SP (Cheyne, 2003).
Currently, Recurrent Isolated SP (RISP) is a formal diagnosable sleep disorder, but unfortunately no established treatment for exists, instead clinicians look to first treat associated neurological disorders (Sharpless, 2016; John Fakoya et.al., 2018; Solomonova 2018). “Isolated” incidents are when SP occurs independently of other medical conditions, and when this is identified, doctors approach treatment differently with educating and assuring the patient, providing sleep hygiene and insomnia advice, and cognitive behavioural therapy – these include not sleeping in a supine position, no caffeine before bed, and relaxation techniques. (Sharpless, 2016). And finally, if you ever experience an SP episode, now you know it is all in your head.
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