What really goes bump in the night?

Imagine waking up to a dark figure above your bed, feeling the heat of its breath on your face and the pressure of its claws on your shoulders. You try to move but your muscles are locked in place. You try to call out, but you can’t muster a sound. If an experience like this sounds familiar, you have probably encountered a terrifying spell of sleep paralysis (SP). Sleep paralysis is a temporary inability to move or speak which often occurs right after falling asleep. Individuals frequently face disturbing hallucinations as they remain aware of their surroundings, throughout the process. Up to as many as 4 out of 10 people may have SP, but despite how widespread the disorder is, this phenomenon largely remains a mystery.

What happens to our bodies?

During sleep, your brain will temporarily paralyse your body to prevent you from hurting yourself in realistic dreams or acting out if something pushes you to. SP occurs from a little brain glitch that keeps the sensory system in an active state of wakefulness but keeps the body in a paralysed condition. As studies show, it is quite clear what causes the actual body paralysis. Motor cells must receive GABA (the chief inhibitory neurotransmitter in the CNS with a role in reducing neuronal excitability) and glycine molecules from ionotropic and metabotropic receptors in order for paralysis to occur. During SP, the body’s transition from REM (rapid eye movement) to NREM (non-rapid eye movement) sleep is out of sync with the brain. These chemicals are still produced for muscle paralysis, but the brain is in a state of consciousness. Generally, the events occur when a sleep pattern is dysregulated (occurring due to jet lag or frequent naps). However, what is not so obvious is what causes the hallucinations. From most accounts, hallucinations are typically shadowy humanoid figures lurking in the corners of the room. But what makes the hallucinations so similar in the way they play out in the brain?

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What’s up with the shadowy human figures?

According to a study on SP hallucinations, the depiction of the shadowy bedroom intruder is due to a functional disturbance of the right parietal cortex in the brain. This perceived intruder is the result of a hallucinated projection of the genetically hard-wired body image we have (homunculus), in the right parietal region. The same circuits in the brain which dictate aesthetic and sexual preference of body morphology. This would explain why individuals generally see a human-like figure, and not other creatures like giants, small goblins or animals. It should also be noted that a lot of these experiences are typically sexual in nature, which would make sense with this part of the brain being responsible for sexual affinity. Other studies have found that disrupting this region of the brain, using electrical stimulation, can generate the feeling of another person in an individual’s extra personal space, mimicking their body movements. It was predicted that when studying sleep paralysis in individuals with genetically hardwired ‘contortions’ in their body image, for example, those with conditions such as anorexia nervosa, it would be expected for these humanoid figures to have body irregularities too mirroring their own internal body image morphology.

Our perceptions of sleep paralysis can affect our experiences

But interestingly, it has been found that a vivid imagination has the ability to shape SP. Hence, with different perceptions of the experience, the phenomenon can affect people differently. In a study comparing the rates and characteristics of SP in Denmark and Egypt, it was found that Egyptians fear it more. Although Danes put SP down to physiological causes, such as stress, Egyptians interpret SP as a jinn (an evil supernatural being that haunts its victims). It was found that because Egyptian populations fear SP so much more than those in Denmark, not only do Egyptians experience it more than Danes (44% vs 25%) but remarkably also experience longer episodes, three times more often. Based on other findings, such as those from Italy, those who fear SP more experience it more, with stronger effects. Like Egypt, some Italians attribute SP to a supernatural being: this time the Pandafeche creature. They also experienced SP more frequently, had longer episodes and a higher level of fear of the experience than those in Denmark. Fundamentally, there is also evidence that a higher frequency of episodes can lead to longer lasting effects. In a study in Egypt, it was found that college students that have experienced SP have more symptoms of PTSD, trait anxiety and pathological worry compared to those who have not. Those who believe SP to have a supernatural root are the most at risk. Under the lens of fear, SP considerably worsens.

Altogether, there is evidently a need for a greater awareness of the physiological and psychological root of SP. This is not only necessary amongst the public, but particularly amongst health professionals so that the anxiety and stress associated with it can be minimised with diagnosis. Furthermore, although difficult to disseminate information across to those with age-old religious beliefs, providing scientific evidence to those in cultures governed by religious ideas could drastically reduce the trauma associated with sleep paralysis. Although the root of sleep paralysis is not malevolent, the myths associated with it can be deeply poisonous.

Where to find out more: 

• How our muscles are paralysed while we sleep: https://www.sciencedaily.com/releases/2012/07/120711131030.htm#:~:text=Brain%20cells%20called%20trigeminal%20motor,neurons%20caused%20REM%20sleep%20paralysis.
• The “bedroom intruder” and what our hallucinations mean: https://www.sciencedirect.com/science/article/abs/pii/S0306987714003600
• Another paper on that shadowy figure: https://www.nature.com/articles/443287a
• Sleep paralysis in Denmark vs Egypt: https://link.springer.com/article/10.1007/s11013-013-9327-x
• Sleep paralysis in Italy: the Pandafeche: https://journals.sagepub.com/doi/10.1177/1363461520909609
• The effects of Sleep Paralysis: PTSD and anxiety: https://pubmed.ncbi.nlm.nih.gov/26488914/