Imagine waking up in pitch darkness, paralyzed and unable to speak. You realize with a start that there is a monstrous fanged cat on your bed, and then the creature attacks you.
This sounds like something out of a horror movie, but it is an actual sleep paralysis case that my colleagues and I reported in a study conducted in Italy.
Paralysis during REM sleep is a physiological phenomenon with a protective purpose: to prevent our sleeping selves from acting out and getting hurt (or hurting someone else). The brain temporarily paralyzes our entire body via a tightly controlled release of chemicals that tilt us between sleep and wakefulness. Almost all of us experience such paralysis when we sleep. But as many as one in five people, at least once in their life, wake up while they’re still in REM paralysis. This condition, sleep paralysis, is where the nightmarish visions that my colleagues and I study come in.
As if being paralyzed in one’s bed weren’t chilling enough, those who wake to find themselves in that state may also report encountering terrifying bedroom intruders, such as witches, demons, and vampires. Some report being endowed with spooky new abilities, like being able to float outside their body or gaze down upon their sleeping self from the bedroom ceiling. Some cultures believe these experiences are a type of astral travel during which the spiritual self leaves the physical body, journeying into a mystical alternative dimension.
My colleagues and I, based on nearly a decade of research, have a more down-to-earth explanation.
Nearly 40 percent of the people who experience sleep paralysis report terrifying and sometimes violent encounters with apparitions that seemed real to them. They often report seeing creatures that close in on them and choke or suffocate them, a sensation that is actually just the shallow respiration of REM sleep.
Neuroscientist V.S. Ramachandran and I propose that the reason for these vivid visions is related to brain structures that gives amputees and those born missing a limb the sensation of having one, a so-called phantom limb. Research into phantom limbs suggests that we come hardwired with a body map, regardless of whether all our parts are present and accounted for. This same internal map may be connected to emotional and visual centers in the brain. During sleep paralysis, disturbances to one’s sense of self, or body image, may occur.
Rather than close encounters with phantasmagoric creatures, then, the terrifying visions of the sleep-paralyzed may be the result of natural disruptions to the brain’s ability to construct a unified sense of self, the feeling of being anchored in the present in one’s own body.
Out-of-body experiences, which also occur in waking life, albeit rarely, can be reliably reproduced in the laboratory. We simply disrupt the region in the brain that enables the ability to distinguish between “self” and “other.” That region, the temporoparietal junction, is located in the top-middle part of the brain. Its function is switched off during REM sleep, which is why in dreams your sense of self is loosened up. You might see yourself from a third-person perspective or find yourself suddenly inhabiting another person’s body.
When one is aware of being paralyzed in sleep, the brain’s motor cortex, which is important for movement, sends signals to the body to move. The motor cortex also sends messages to the parietal lobes, which monitor the neurons that are firing commands to the limbs to move. But the limbs can’t move, because they are temporarily paralyzed.
These conflicting signals during sleep paralysis affect the brain’s ability to construct a sense of self. The brain will try to clear the confusion by building a body image. It’s the brain equivalent of predictive text, which anticipates a word as we type but often gets it wrong. In a similar way, the brain tries to complete an image but gets it wrong. The result is bizarre hallucinations in which a sleeping person might “see” their limbs rotating in the air like a tornado or their body sinking deep into the bed as if disappearing in quicksand.
As it perceives that movement, the brain continues to send messages to the paralyzed limbs to move. The paralyzed arm, say, cannot send a message back to the parietal lobe that it can’t move, so the sleeper may hallucinate the presence of an arm. The brain and our visual system are prediction machines, always in the business of creating internal models of what they think will happen next and then trying to meet those expectations.
So how does this all lead to seeing a ghost? There are three things we think are important:
First, neurons important for empathy and understanding the intentions of others may be involved. During our waking lives, we can imagine the world from another’s perspective, but it doesn’t create an out-of-body experience because we’re still getting feedback from our limbs and frontal brain regions. We thus feel anchored in our own body while differentiating it from that of the other person. But during REM sleep, there is no sensory feedback from one’s limbs, and the frontal brain is weak, so the neurons in the brain regions that help distinguish between one’s self and others — important for understanding others’ intentions — become hyperactive, breaking down that self/other barrier. So merely imagining a bedroom intruder might produce a hallucination of one, which to the sleep-paralyzed seems real.
Second, chemical changes in the brain could further facilitate these fantastical visions. Serotonin, famous for its role in mood, is key for waking up the sleeping brain under normal circumstances. But during sleep paralysis, a massive intrusion of wakefulness into REM sleep may flood the brain with serotonin, exciting a receptor that is also triggered by psychedelic compounds like LSD and psilocybin. Intense activity in this serotonin doorway heightens one’s inclination to ascribe meaning to the otherwise meaningless. It also drives fear. Indeed, it creates just the right chemical environment for the brain to turn a physiological event like sleep paralysis into a petrifying otherworldly experience.
Finally, the brain has a tendency to connect events in the world. It is both a statistical machine and a savvy storyteller. The inability to speak and physical paralysis, coupled with crushing chest pressure, can be conveniently spun into a coherent story once the sleeper has truly awoken: A ghost was in my bedroom and it was trying to suffocate me.
Indeed, an experiment has found that when the temporoparietal junction is disrupted using an electrical current, a person who is wide awake will sense a shadowy figure, a ghost-like doppelgänger, which stands behind them, mimicking their gestures.
We’re now, for the first time, beginning to explore the neurological basis of enigmatic ghost visions that have puzzled people throughout history. Not having to worry about ghosts may help us all sleep more soundly.
Baland Jalal is a researcher in the department of psychology at Harvard University and a visiting researcher in the department of psychiatry at the University of Cambridge, in Britain.