The Science Behind Sleep Paralysis

I recently listened to a podcast where the topic of ‘sleep paralysis’ came into conversation. Having never experienced the phenomena myself, but having many friends mention their brush with this paralysis, naturally I was curious to investigate further. A 2011 review showed that around 7.6% of the global population will experience at least one sleep paralysis episode in their lifetime, although there were higher rates recorded in students and psychiatric patients, particularly those with PTSD or a panic disorder.

So, what is sleep paralysis? From a non-scientific perspective, sleep paralysis locks a person in a temporary state between waking and dreaming, where they can't move but may experience odd hallucinations. People have been known to have a number of bizarre experiences from feeling pressure on their chest or a hand around their throat to a feeling of being removed from their frozen bodies, as if floating out of bed. This is sleep paralysis, a fairly common and diagnosable sleeping disorder.

Just why or how it happens isn’t fully understood yet. From a scientific standpoint, sleep paralysis is better understood now to be a neurological disorder, rather than a brush with the paranormal, that researchers believe arises from disrupted REM sleep. As a person falls asleep, their heart rate slows, muscle activity decreases, and brainwave activity slows in frequency. This is non-REM sleep. A normal sleep cycle involves a stage called rapid eye movement (REM); a unique phase of sleep in mammals and birds, distinguishable by random/rapid movement of the eyes, accompanied with the paralysis of voluntary muscles, and the tendency of the sleeper to dream vividly. It is during REM that about 90% of dreams occur. During REM sleep, we have three systems engaged: 

1. Cortical activation – the brain is switched on creating the dream imagery.
2. Sensory blockade – senses are blocked meaning no sight, no hearing and you stop feeling your body in the bed.
3. Muscular paralysis – structure of the brain called ‘pons’ which sends signals to the spinal cord which paralyses the major muscle groups while you dream

Usually when you wake up, all three REM systems become disengaged at the same time. Sometimes, due to influencing factors such as jetlag, recreational drugs, sleep deprivation, or sometimes it just happens, two systems switch off while one stays engaged. When you wake up, cortical activation and sensory blockade are now switched off (we are awake, thinking hearing seeing etc.) while muscular paralysis stays on. The only body parts not paralysed during REM sleep and during this period are the respiratory system and eyes. Your eyes can look around and the respiratory system - because it is the only part not paralysed - is the only thing you can ‘feel’. Because of this, some people experience an intense pressure on their chest, pushing down and it feels like your breathing is very shallow because you can effectively feel the respiratory system in action. Your brain begins to think ‘well, if two of these systems are disengaged, but not three, maybe that means I should still be dreaming?’. In turn, the dreaming part of the brain switches back on; this happens whilst your eyes are open, so the imagery is referred to as a ‘psychotic hallucination’. So now suddenly you’re paralysed, experiencing pressure on your chest and you’re starting to hallucinate… and this is supposedly a normal and common disorder?!  Sometimes we hallucinate the last thing we were dreaming about.

Let’s take an example:

Say I’m dreaming about my dog (Alfie, a rather cute and fluffy spaniel in case you were curious). Suddenly my dog is sat at the foot of my bed. I love Alfie, but why is he in my room? This freaks me out and I feel scared; the brain responds to the emotional atmosphere of being freaked out and changes the hallucinations to fit. Now suddenly its not a cute fluffy spaniel, it’s a ferocious wolf bearing its teeth. Suddenly the wolf is linked to the feeling of pressure on the chest so now there’s a ferocious wolf pushing down on my chest and I’m having a full-on freak out.

That’s one possible explanation to the science behind sleep paralysis. Another is presented below by Daniel Dennis, a postdoctoral scholar in psychiatry:

"We know the amygdala is highly active in REM, which is important to fear and emotional memory. You have part of the brain actively responding to fear or something emotional, but nothing in the environment to account for that. So, the brain comes up with a solution to that paradox."

So how common is this? Up to as many as four out of every 10 people have some experience with sleep paralysis. A study in the Sleep Medicine journal of 185 patients diagnosed with sleep paralysis highlighted that around 58% sensed a non-human presence in the room with them and 22% saw a human in the room, typically a stranger.

What are the factors that cause sleep paralysis? Sleep experts believe that it may be partly genetic, but other factors include a history of trauma, poor physical health and sleep quality or substance abuse. The frequency and severity of episodes has also been linked to anxiety-like symptoms and sleep deprivation – this may explain why episodes come and go. They may coincide with periods of stress or anxiety.

Is it treatable?

Medication wise, no, there is no direct treatment for ‘sleep paralysis’. A routine change is usually advised to patients, including an improvement to sleep schedules and to maintain a better and more consistent bedtime routine. According to the National Health Society of the UK, in more extreme cases, patients may be prescribed a low dose of antidepressants. These medications may help lessen the symptoms of sleep paralysis by suppressing certain aspects of REM sleep. 

References:
https://www.youtube.com/watch?v=Hw7NFoKOIu4 
https://www.webmd.com/sleep-disorders/features/sleep-paralysis-demon-in-the-bedroom#1

https://www.livescience.com/50876-sleep-paralysis.html