Is this folklore or is there science behind it?

We're going to hash it out in this video and talk you through what we know about sleep paralysis.

Sleep paralysis has literally been the stuff of nightmares for thousands of years.

Stories of this phenomenon can be found all across the world.

With supernatural undertones passed on through traditional fairy tales, Canadian Eskimos believed that manic spells could paralyse people and induce hallucinations during their sleep.

In Japanese culture, kanashibari, kane meaning metal and shibari meaning to bind or tie, is associated with a sleep demon that strangles the victim while they sleep.

Similar tales originate from Brazil, Spain and China describing shapeless creatures, intruders and ghosts as the perpetrators.

More modern interpretations are those of alien abductions, often featuring humanoid beings with large bug eyes, carrying out medical or even sexual experiments on their abductees.

This is some scary stuff but let's take a deeper look into the science and the evidence.

So what is sleep paralysis?

These are more commonly known as night terrors and they're a part of a group of sleep disorders called parasomnias.

These conditions manifest as behavioural disorders associated in particular with waking up during sleep.

Episodes of sleep paralysis in particular occur just as you're about to doze off or wake up, when you expect it the least.

If you think this is a rare phenomenon, think again.

Nearly one in ten people in the general population will experience sleep paralysis in their lifetime.

Despite this, our definition and understanding of the disorder is poor.

But what do we actually know?

Sleep paralysis can happen in healthy individuals as a one-off event.

But ongoing episodes have been linked with underlying psychiatric, hereditary and other sleep disorders.

It usually doesn't last longer than a few minutes at a time and it can be linked with the dream-filled portion of sleep, otherwise known as the rapid eye movement or REM phase of sleep.

This allows your eyes to continue moving as normal together with your chest muscles that help you breathe, whilst other voluntary muscle movements like your limbs or your arms and your legs are stopped or inhibited.

When REM-based atonia or paralysis intrudes into wakefulness, it causes the dream world and the real world to collide, producing terrifying illusions, hallucinations and sensations.

People who experience sleep paralysis report three different types of hallucinations.

The first is the incubus.

This is the feeling of weight or pressure on the chest whilst being choked or suffocated.

The second sensation is that of an intruder in the bedroom.

Traditionally, this would be some kind of malevolent presence or spirit, which is personalised and interpreted through visual, tactile and kinetic cues.

And lastly are vestibular motor hallucinations.

That's the feeling of dissociation or to you and me, an out-of-body experience.

Before we continue, it'll be useful to catch up on the neurobiology that supports REM sleep.

Don't forget we've covered some of this in our Anatomy of Dreams video.

But anyway, let's go into the specifics of sleep paralysis.

There's an area at the back of the pons in the brain called the sublateral dorsal nucleus.

We'll call it the SLD nucleus from now on.

But it's literally a nucleus that's at the back, to the side and just under the pons.

And this pontine circuit plays a critical role in muscle atonia during REM sleep.

That's the paralysis part and is comprised of some really complex neurocircuitry.

In particular, increased firing of specific SLD neurons is associated with REM sleep and causes your muscles to become atonic or paralysed.

These neurons give us clues towards understanding what goes wrong in something like sleep paralysis.

Currently, research indicates a strong link between REM sleep disorders and the development of neurodegenerative diseases like Parkinson's and other types of dementia.

This suggests that REM sleep disorders may be characterised by the loss of neurological mechanisms that control your REM sleep.

Now it's important to note that just because you're getting sleep paralysis experiences does not mean you're developing Parkinson's disease or some kind of dementia.

Lots of things can make these circuits fire wrongly.

Some of the risk factors including drinking way too much alcohol and being sleep deprived.

And that's why people commonly experience episodes of sleep paralysis after a long-haul flight, for example, when they're in a new time zone and they're sleep deprived.

So, there's a neural reason why you might stop being able to move while you're waking.

But why do people experience the hallucinations that go with it, specifically that incubus or the out-of-body sensation?

The incubus sitting on the person's chest or the malevolent spirit in the corner of the room that's watching you is thought to be attributed to a disturbance in the parietal cortex, an area of the brain responsible for interpreting sensations and perceptions and integrating what you're seeing, hearing, feeling and touching with your visual system and plugging that all together.

This normally allows you to construct spatially coordinated images that project and integrate with your natural environment.

Now, your muscles are a really integral part of this mechanism and they give your brain some input about what's going on in and around the world around you.

But in this case, your muscles are paralyzed but you're waking up or dozing off.

Without this important sensory integration, centers including the right superior parietal lobule and the In case you've never heard of it, this is something we call the somatosensory homunculus.

It's organized by neural pathways that communicate between our sensory organs and the brain.

It lays across the cortical surface of the parietal lobe and in pictures and textbooks, it often looks like a man draped over the brain and it assigns small sections of the sensory cortex to equivalent areas of your body.

This is how the brain processes sensation and may explain why people with sleep paralysis misattribute human-like shadow illusions to real ghosts and supernatural beings that really feel like they're standing right there.

That's all well and good but what about the vestibular motor or out-of-body experiences?

That's slightly trickier to understand and is completely theoretical.

To try and figure this out, we need to talk about serotonin and LSD.

Serotonin is an important neurotransmitter that's implicated in mood.

Looking at well-established research and findings, serotonin has a really integral role in visual processing and that's all linked to a receptor in the brain called 5-HT2A and that's an important piece of this puzzle.

As with a lot of our understanding about the brain, we need to look at when the brain goes wrong or when it's diseased.

Both schizophrenia and Parkinson's patients show increased densities of these serotonin receptors in the visual cortex and also report experiencing really odd visual hallucinations or delirious episodes.

To add to that, meditation and drug-induced altered states of consciousness are known to cause mystical and intense hallucinations, very similar to what people experience in sleep paralysis.

We know that the dissociative feelings or the out-of-body experiences caused by LSD through the 5-HT2A receptor activations may lead to users of LSD attributing increased meaning and personal involvement with otherwise meaningless things like shapes or sounds in the air.

This characterises something called personal relevance attribution and that may embody some of the aspects of personal narrative and the out-of-body experience described in sleep paralysis.

Overall, this is a hugely interesting and complicated area of research that's, again, only really at its infant stage of exploration, even more so than the anatomy of dreams.

I know this was a really whistle-stop tour jumping from culture to neurobiology, functional anatomy and even psychopharmacology but I hope it's given you the impetus and I hope it's made some slight sense of this really complex matter and inspired you to go and explore for yourself.

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