Sometimes you wake up in the
middle of the night and can't
move — here is the scientists explanation

Written by CHRISTINA STERBENZ

What is sleep paralysis?

Every few months I have a terrifying experience
in the middle of the night.
I wake up but can't move, except for my eyes,
which dart frantically underneath fluttering,
heavy lids. I feel a heavy presence on top of my
chest, squeezing the air from my lungs and
throat. Then a shadowy, cloaked figure starts
looming just within the corners of vision.
I'm not dreaming. And no matter how many
times it happens, the panic sets in. As a kid, I
thought the devil had paid a visit to my
bedroom.
Now I know these symptom stem from a strange
sleep phenomenon called sleep paralysis. While
various social and psychological factors can
influence the prevalence of sleep paralysis, a
2011 paper combined 35 studies with more than
36,000 participants total. The authors found that
7.6% of the general population experiences
sleep paralysis, rising to 28.3% in high-risk
groups, like students who have a disrupted sleep
pattern. And in people with mental disorders,
like anxiety and depression, 31.9% experienced
episodes.
"When you're experiencing sleep paralysis, you
become conscious," Daniel Denis, a PhD
candidate in cognitive neuroscience and
researcher at the Sleep Paralysis Project , tells
Business Insider. "The idea is that your mind
wakes up but your body doesn't. "

Why you can't move

Sleep has three or four stages of non-REM (rapid
eye movement) sleep and one REM state. While
people can dream in any stage, REM is the most
closely associated with vivid dreams, the type
that seem real.
The brain also stays active during REM —
"almost comparable to during the day," Denis
explains. People naturally become paralyzed
during REM, probably to prevent themselves
from acting out their dreams, a process known
as REM atonia.
Many who wake up during this state simply
open their eyes and quickly begin to move
around. But those suffering from sleep paralysis
experience "a sort of failure of the molecular
clock," as Denis puts it. For whatever reason,
REM atonia continues after you've waken up.
Most episodes last a few seconds to a minute,
but in much rarer cases, people can require 10
to 15 minutes before they fully regain motion.
About that shadowy friend of mine —
researchers don't have the best explanations for
he/she/it. To start, I could be experiencing my
brain's interpretation of myself. The parietal
lobes may be monitoring the neurons in my
brain telling my limbs to move, according to a
study from UC San Diego, published in the
journal Medical Hypotheses . Since they can't,
the brain hallucinates the intended movement.
Denis explains that the "intruder" might also be
due to an over-active amygdala, a part of the
brain responsible for fear (among other things).
"You wake up with your amygdala screaming,
'There's a threat!'" he explains. "So your brain
has to invent something to fix the paradox of
the amygdala being active for no reason." While
the amygdala remains active during REM sleep,
total paralysis right after awakening can send it
into overdrive.
The experiences
One of the first in-depth studies on sleep
paralysis in 1999 defines the three main
categories of sleep paralysis hallucinations as
the "incubus," the "intruder" and "unusual
bodily experiences."
In the first case, people feel an intense pressure
on their chests, inducing the feeling that they
can't breathe.
As the authors note, sleep paralysis affects only
the "perception of respiration." Breathing is
reflex-based, so nothing truly separates these
poor few from the oxygen they desperately
need. It just feels that way because they're
afraid.
“When you’re in REM, your breathing is very
shallow and your airways become quite
constricted, so it would be difficult to breath
anyway," Denis explains. "But when you become
conscious of that, it can be terrifying."
People experiencing the second category, the
"intruder," can feel a "sensed presence, fear,
and auditory and visual hallucinations," the
researchers note. Essentially, your mind invents
a vision to solve some sort of paradox in the
brain that occurs during sleep paralysis. The
authors describe it as a "hypervigilant state of
the midbrain," which can make people highly
aware of even the smallest stimuli and "biased
toward cues for threat or danger." That's why a
small sound can seem horrifying to someone
experiencing sleep paralysis.
The intruder and the incubus go hand-in-hand.
Both symptoms typically involve the threat-
activated systems in the amygdala , as
mentioned earlier. Some people even relate the
"intruder" and the incubus, reporting that they
feel someone strangling or suffocating them,
Denis says. But the third type of sleep paralysis
hallucination, the "unusual bodily experiences"
are the least common.
When people experience "unusual bodily
experiences," they often feel like they're having
an out-of-body experience, levitating or flying
around the room, as the 1999 study explains.
This third type appears to be associated with
REM stages where the brainstem, cerebellar,
and cortical vestibular centers are activated,
according to a 2013 study of 133 patients with
panic disorder .
The pons, which inhibits movement during
sleep, falls into that area, Denis notes. "You feel
like you're moving when you're not because the
area of the brain that coordinates that is
overactive," he says.
Myths and folklore
Cultural beliefs also strongly influence these
hallucinations and experiences, leading to the
creation of folklore and myth, which can blur
fact with fiction. The " Old Hag," for example, is
the primary interpretation of sleep paralysis in
Newfoundland. And similar fantastical stories
exist about the Boto, a pink river dolphin in the
Amazon Basin that transforms at night into a
lustful prowler, explains " The Devil in the
Room," a documentary that explores the
paranormal and mythical aspects of sleep
paralysis.
Take a look at Henry Fuseli's 1781 oil painting,
"The Nightmare," shown below, thought to be
one of the clearest artistic interpretations of
sleep paralysis.
Personally, my over-active amygdala conjures
images of the devil — unsurprising considering
I live in a majorly Christian nation and grew up
mildly Catholic. From his research, Denis says
that "modern Western culture" tends to see
burglars, rapists, and aliens.

Prevention

While sleep paralysis can be hereditary, it can
happen to anyone. Factors like lack of sleep,
sleep disturbances, jet lag, and shift work can
increase someone's likelihood of experiencing
it, and certain groups, like African-Americans ,
can also experience it more commonly. Sleep-
paralysis episodes have been linked to
hypertension, seizures, and narcolepsy, a sleep
disorder where people lose their ability to
regulate sleep cycles and can fall asleep at
random and unexpected moments.
While stress, anxiety, and depression often
trigger the episodes, we can't exactly control
these factors. So beyond trying to reduce stress
and getting plenty of sleep, how can you
prevent the terrifying onset of sleep paralysis?

Avoiding sleeping on your back could help.
Research has shown that people that experience
sleep-paralysis episodes are three to four times
more likely to occur in people who sleep in the
supine position. Some people even use
nightwear that makes lying on their back
uncomfortable, according to Denis.
But if you do wake and find yourself unable to
move, focus all your energy on wiggling a toe or
finger. "So long as you can move one muscle,
that breaks the paralysis," Denis advises.

Informative.