Causes of Sleep Paralysis

Why it happens, and what makes it such a frightening feeling.

Long Article

Sleep paralysis is the feeling that you can't move, while seeing dream-Like hallucinations.

What is Sleep Paralysis?

Sleep paralysis is a “parasomnia” (sleep disorder) during which the sufferer feels awake but can’t move. It often comes with hallucinations that can be seen, heard, or felt, and intense feelings of fear. It can happen only once, or many times to the same person, with about 5% of people experiencing sleep paralysis on a regular basis. It’s not considered physically dangerous, but it can be really unpleasant and frightening. Sleep paralysis has a long, documented history, with recurring themes in how the hallucinations tend to present themselves, including floating, choking, and a feeling of strangers in the room or on the bed.

What Causes Sleep Paralysis?

There are a few theories about what triggers sleep paralysis, but the most likely cause is an overlap between the REM (rapid eye movement, dreaming) part of sleep and the next step of sleep where you begin to wake up. To see why this would happen, and how it causes the symptoms of sleep paralysis, we’ll need to take a look at how the different parts of the sleep cycle operate.

A Full Night’s Sleep

Over the course of the night, your body goes through 4 to 6 cycles of NREM (non-rapid eye movement) and REM sleep, each of which last around 80-120 minutes. This cyclic motion starts with NREM-1 (N1), which is the first part of falling asleep. During N1 your muscles can still be active, your eyes may open and close, and there’s a general feeling of drowsiness.

Stage N2 is the first real stage of sleep, where the muscles continue to relax, and outside sounds lose meaning. Someone in N2 sleep can still be woken up easily, and dreaming is very rare. From there, N3 sleep, or deep sleep, takes over and you’re hard to wake up, with occasional mild and disjointed dreams. Unlike REM sleep, the muscles in the body aren’t paralyzed in NREM, and acting out dreams, sleepwalking and a few other parasomnias can be a problem during the N3 part of the cycle.

REM sleep is the big one for dreaming. The motor neurons are switched off, causing paralysis, your breathing changes, and the body can no longer regulate its temperature as well. Lucid, vivid, and dramatic dreams are most likely to happen in REM, and the muscle paralysis is part of what keeps us safe and away from acting out dreams as they happen.

Modified from RazerM: Hypnogram of sleep between midnight and 6.30 am

In a normal sleep cycle, you should start with N1, then cycle N2->N3->REM->N2->N3->REM and back again until you finally begin to wake up, with the REM part of the cycle gradually getting longer over time. It’s normal to switch up the order in the middle and go from N2 to REM and back, and to briefly wake up and fall back to sleep during REM, usually with little or no memory of it happening.   

Enter Sleep Paralysis

A likely cause of sleep paralysis is the blurring of REM sleep and the process of waking up. There are a couple of theories around why this might happen, the main ones being:

  • Sleep happens with fast, choppy REM cycles: People who have sleep paralysis have shorter “REM sleep latency.” This means that it takes less time for them to enter REM sleep than for most people going through the sleep cycles. They also have shorter NREM sleep, and fragmented REM, meaning their REM sleep is broken into shorter segments and frequently disrupted. This disturbance of normal sleeping cycles could help cause REM sleep and wakefulness to overlap.
  • The balance of sleep-wake chemicals is wrong: The chemicals to make you stay asleep (related to acetylcholine) are doing too much, and the chemicals to make you wake all the way up (related to serotonin) aren’t doing enough. Because of this imbalance, there isn’t enough of the “awake” chemical to actually wake you up, but you’re not fully in REM sleep, so you’re not asleep enough to block out external sounds, movements, etc.

Either one of these theories could help explain the common signs and symptoms of sleep paralysis, which we’ll look at now.

Sleep Paralysis Causes

Paralysis

If sleep paralysis is an overlap with the REM cycle, this one is pretty easy. The normal process (called REM atonia) that inhibits motor neurons and immobilizes the body during REM would also prevent people from moving here. While this is bad for sufferers of sleep paralysis, not having this response can also be awful – people with REM behavior disorder can’t turn off muscle movement and act out their dreams, sometimes with loud or violent consequences.

Hallucinations

There’s no agreed upon theory for exactly what causes the vivid hallucinations, but it could be tied to dreams, vigilance, or an over-active fear response. Because sufferers have some ability to hear and feel pressure, a hyper-vigilance is activated in the brain that creates hallucinations in response to the input that it does get, trying to make sense of the situation. Combined with panic, this emergency response could contribute to the sense of intense dread felt by many people with sleep paralysis.

Feeling of Choking and Suffocation

One of the characteristics of REM sleep is rapid, shallow breathing and an increased likelihood of a slight blockage in the airway, especially for people who sleep on their back (sleep paralysis is also more common for back-sleepers). Since people experiencing sleep paralysis can’t directly control their breathing, stress and panic combined with shallow breath can be interpreted as pressure or suffocation, even if enough air is coming in. This can also be linked to the feeling of pressure or that an entity is sitting on the chest.

Temperature Drops

Feeling an ominous chill is another common sign of sleep paralysis, and it could be tied to the body’s poor temperature regulation during REM. Because of the muscle paralysis, the body can’t shiver or change breathing patterns to warm itself up, so there’s no real way to mitigate that chilling feeling if the room is cold.

Why Does Sleep Paralysis Only Happen to Some People, and Not Everyone?

Who ends up with sleep paralysis and how often seems tied to a combination of genetics and sleep hygiene. It does run in families – twin studies have shown that if one twin experiences sleep paralysis, it’s very likely the other one will as well. Poor sleep is also a big factor; insomnia, sleep deprivation, and inconsistent sleep patterns call all make sleep paralysis more likely and worse. This can be a self-perpetuating cycle – people are stressed about having sleep paralysis, which makes their sleep worse, making episodes of paralysis more likely. Overall, it probably happens at least once for about 8-20% of the population, and chronically for around 5% (different studies have found wildly different rates for the general population).

There are a few groups with a higher than average risk. People who use mental health services, especially those diagnosed with a panic disorder, are more susceptible, and so are students. This is probably because both groups are more likely than average to have disrupted sleep.

People with consistent, unbroken sleep and low genetic risk don’t seem to have the right combination of factors to trigger a sleep paralysis episode.

Why Aliens and Shadow People? Why Are These Themes So Common?

There’s a huge cultural element to the content of sleep paralysis hallucinations. Sleep paralysis is actually pretty well documented in historical contexts, and what shows up is very strongly tied to people’s beliefs. While there are some common feelings (floating, pressure on the chest, figures in the room), the entity causing these feelings depends on when and where you are – it was witches then and aliens now, because that’s what’s people talk and hear about. We’ll be covering sleep paralysis themes over time and across cultures in our next article. Stay tuned!

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