The liminal space between sleeping and waking could be where the human brain conjures the uncanny.
It’s there, in a state that is somehow both or neither, that the sleeping brain may extend tendrils into consciousness, or the waking mind jostle uncomfortably at the edges of sleep.
Such overlap, researchers have now found, may be behind some of the strangest experiences humans report, from the terrors of sleep paralysis to the apparent ability to send one’s mind away from the body and wander around without it.
And, in turn, that could tell us something about the very nature of how we experience living in our bodies.
Not everything we experience is easy to explain. We understand itching, and hunger, and jealousy, for example; but some people report sensations or experiences that do not match what we know the body can do.
Some people, for example, report floating or flying sensations; others report intense dread and monstrous presences; while others still report the eerie sensation of leaving their own bodies behind to wander through physical or spiritual spaces.
Scientists refer to strange distortions of bodily perception as unusual bodily experiences, or UBEs.
They can occur during sleep paralysis, lucid dreaming, and even while a person is awake.
And no one really knows why they happen.
But since they so often seem to happen around the boundaries between sleeping and waking, neuroscientist Teresa Campillo-Ferrer of Radboud University in the Netherlands and her colleagues devised a cunning plan to gather more information.
They recruited 35 people who had some experience with the stranger reaches of consciousness through lucid dreaming or meditation, and recorded their brain activity as they ventured into the hazy borderlands between sleeping and waking – during meditation, while drifting off to sleep, and even while dreaming.
Their equipment was rather less mysterious: EEG caps fitted with dozens of electrodes to record the electrical activity of the brain.
Caps donned, the travelers were dispatched to their preternatural roaming.
Each participant was guided through meditation exercises designed to help them relax and focus on their bodily sensations. The researchers also used flashes of light to encourage unusual experiences as the participants drifted into sleep.
But there was a slight snag. How do you tell a scientist you’re having an out-of-body experience when you’re asleep?
The researchers had a clever solution. They instructed the participants to move their eyes in a deliberate left-right-left-right sequence whenever they experienced something unusual.
Because eye movements can be detected even during sleep, this gave the team a way to pinpoint when the experiences were taking place in the EEG recordings.
Afterward, they interviewed the participants in detail about what they had felt, seen, and experienced, allowing the researchers to match their subjective accounts to the electrical activity recorded in their brains.
Of the 35 participants, 20 reported 36 UBEs between them – the largest number during meditation, but others during REM sleep, non-REM sleep, and sleep arousals – brief periods of emerging from sleep into wakefulness.
Some participants felt themselves floating, falling, or spinning, even though their bodies remained perfectly still. Others experienced strange vibrations, pressure, or the sensation of being touched by something that wasn’t there.
Some lost their sense of where their bodies ended, or even what shape they were. Others experienced paralysis, sometimes accompanied by frightening hallucinations. In 10 cases, participants reported the sensation of being outside the physical body, or somehow elevated above it.
And, in the brain activity of these experiences, an intriguing pattern emerged.
Compared with the participants’ normal brain activity during sleep or meditation, the UBEs were associated with a decrease in slower brainwaves, called delta and theta waves, and an increase in faster beta and gamma waves.
The changes were particularly pronounced around the temporal regions of the brain, which help process sensory information and contribute to our sense of our own bodies.
Here’s where that pattern gets interesting. The slower brainwave activity appeared to fall somewhere between the patterns typically seen during sleep and wakefulness, while the faster activity more closely resembled that of a waking brain.
In short, the brain activity associated with these strange experiences seemed to combine features of both sleeping and waking, regardless of whether the participants were technically asleep or awake.
The researchers believe that this may be because the brain involved has poor boundaries.
For an awake brain, this means the sleeping brain is intruding on its turf; and for a sleeping brain, wakefulness is seeping in around the edges.
A similar overlap has long been implicated in sleep paralysis – consciousness returns while the body remains temporarily paralyzed by sleep – but its role in other strange bodily experiences is less well understood.
“Taken together, our findings suggest that UBEs may not depend on a specific sleep stage or waking condition, but rather on a state of consciousness that blurs the boundaries between them,” the researchers wrote.
“This could explain why UBEs sometimes integrate elements of dream-like experiences with the external environment in which participants are located, such as seeing one’s bedroom while experiencing fearful hallucinations during sleep paralysis episodes.”
There are still mysteries to unravel. The EEG recordings can’t reveal whether the brain as a whole enters an intermediate state, or whether different regions are simultaneously engaged in sleeping and waking activity.
Further research is needed to investigate these possibilities and determine exactly how the brain constructs illusory sensations that feel so real.
But the findings could offer clues to something even more fundamental – how the brain establishes our sense of being anchored deeply inside a body in the first place.
The research has been published in Consciousness and Cognition.
This article was fact-checked by Rebecca Dyer and edited by Rebecca Dyer. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.
