For hundreds of years, people of all ages, from all over the world, have told similar stories about moments their lives were in real peril: an intense white light, a sense of tranquility and feeling as if they were somehow floating above their body.
For neurologists, these so-called near-death experiences have a neural basis, one that may, according to new research, be similar to what goes on in the brain during certain sleep disturbances.
"I have a theory that near-death experiences may occur when the brain is still functionally and structurally intact," said Dr. Daniel Kondziella, a neurologist at the University of Copenhagen.
Kondziella is the lead author of new research on near-death experiences that is being presented Saturday at a meeting of the European Academy of Neurology Congress. His findings, which have not yet been published in a peer-reviewed journal, suggest that the typical hallmarks of such episodes, such as bright white light and a sense of tranquility, are most likely the result of neural activity in the brain, similar to what goes on during a phenomenon called sleep paralysis.
"I do think these experiences can be triggered in situations with imminent death," Kondziella told NBC News. "But while perceiving these experiences, brain networks are working to be able to store those experiences, to be able to be resuscitated, to retrieve those memories, and to tell us about them."
"I think that before passing out, they have the near-death experience. When they are then resuscitated, the last thing that they remember is that experience," he said.
Near-death experience and sleep paralysis
Kondziella's study was based on questionnaires given anonymously to 1,034 people online. The questionnaires began with a single query: Have you ever had a near-death experience?
The definition of such an experience was broad: "Any conscious perceptual experience, including emotional, self-related, spiritual and/or mystical experiences, occurring in a person close to death or in situations of intense physical or emotional danger." And because the responses came from anonymous individuals, it was impossible for the researchers to confirm any of the responses.
Those caveats aside, the researchers found that 106 people, or about 10 percent of the respondents, reported what was considered to be a "true" near-death experience.
What's more, participants who reported near-death episodes were also more likely to have a history of extreme and vivid sleep disturbances, referred to as REM sleep intrusion (REM is rapid eye movement).
That finding, the researchers said, boosted the theory that these experiences have a neurological basis.
"REM sleep is the kind of sleep where we do most of our vivid dreaming," said Dr. Kevin Nelson, a neuromuscular neurologist affiliated with the University of Kentucky. He was not involved in the study, but published similar research more than a decade ago.
Nelson explained that there are only three states of consciousness in which the brain can exist: awake, non-REM sleep and REM sleep.
Sometimes I wake at night, and I can't move.
During REM sleep, when a person is dreaming, most of the muscles in the body are paralyzed so dreams aren't acted out physically.
Usually, a switch in the brain allows people to move seamlessly from one state of consciousness to another. But sometimes, that switch doesn't work properly, allowing REM sleep and waking to blend.
"So you’ll have elements of the REM system occurring while a person is essentially awake," Nelson said. "People may wake up and they’ll be paralyzed and unable to move. They may have visual or auditory hallucinations. Usually they’re terrified."
This phenomenon is called sleep paralysis, and it was described by some of the respondents in the study. "Sometimes I wake at night, and I can't move," one participant wrote. "I see strange things, like spirits or demons at my door, and after a while I see them coming beside me. I can't move or talk, and they sit on my chest. It scares the hell out of me!"
A 2011 review estimated nearly 8 percent of the world's population has experienced at least one episode of sleep paralysis in their lifetime, and not all experiences are as vivid or scary.
Both Kondziella and Nelson suggest the brain mechanisms responsible for these sleep disturbances also allow people to visualize experiences when their lives are truly in danger, a hypothesis that is bolstered by the study's finding that there is overlap between those who report experiencing both phenomena.
"People whose brains are more likely to blend REM and waking consciousness — under the right circumstances — are much more likely to have a near-death experience," Nelson said.
Nelson also said the part of the brain that allows people to be aware of where they are physically — standing up, lying down, sitting on a chair — is turned off during REM sleep. This may explain in part why some people have out-of-body experiences.
Are near-death experiences real?
Some outside experts were skeptical of the study, and in particular, the finding that one in 10 people have had a near-death experience.
"If anything, I think it’s more likely that people are reporting sleep events erroneously as near-death experiences based on this study," said Dr. Donn Dexter, a fellow of the American Academy of Neurology and a neurologist with the Mayo Clinic Health System.
Kondziella was quick to defend what people experience as near-death episodes, which are often reported to be life-changing and spiritually meaningful.
"As a scientist, I think there is a biological explanation," he said. "But if there is a deeper meaning for them, that is a question for philosophers and religious leaders."
All of the neurologists agreed that there is room for both science and faith in the same conversation about near-death experiences.
"You can be a scientist and still have strong faith," Dexter said. "With science, you have to have a theory, a hypothesis that you can prove false. You can’t prove somebody’s faith false."
"There will always be mysteries that can’t be solved," Nelson added. "But it does no harm to the mystery to know a little bit more about it."