The brain's tendency to occasionally blur the line between sleep and wakefulness may help explain the phenomenon of near-death experience, preliminary research suggests.
It's been an open question as to why some people see bright light, feel detached from their bodies or have other extraordinary sensations when they are close to dying or believe they might die.
Some people view these so-called near-death experiences as evidence of life after death, and many neurologists have considered the phenomenon too complex for scientific study.
But the new research, published in the journal Neurology, implicates the blending of sleep and wake states as a biological cause of near-death experiences.
Researchers found that adults who said they'd had such an experience were also likely to have a history of what's called REM intrusion — where aspects of the dream state of sleep spill over into wakefulness.
People may, for example, feel paralyzed when they first wake up, or have visual or auditory hallucinations as they fall asleep or awaken.
Of the 55 study participants who'd had a near-death experience, 60 percent had also experienced REM intrusion at some point in their lives. That compared with 24 percent of 55 adults who served as a comparison group.
The findings suggest that the brain's arousal system predisposes some people to both REM intrusion and near-death experience, according to the study authors, led by Dr. Kevin R. Nelson, a neurologist at the University of Kentucky in Lexington.
This arousal system, Nelson explained in an interview, regulates not only REM sleep, but also attention and alertness during waking hours — including during dangerous situations.
'Spiritually neutral' research
And many of the features of REM intrusions, he said, parallel those of near-death experience.
During REM sleep, visual centers in the brain are highly active, while the limb muscles are temporarily paralyzed. So REM intrusion during peril could promote the visions of light and sensation of "being dead" that people often have during a near-death experience, according to Nelson.
Other evidence supports a role for REM intrusion in near-death experiences, he said. One important fact, Nelson noted, is that stimulation of the vagus nerve, which connects the brain stem to the heart, lungs and intestines, triggers REM intrusion. And heightened activity in this nerve is sure to be part of the body's "fight-or-flight" response to danger.
Still, Nelson said he doesn't think REM intrusion will turn out to be the "whole explanation" for near-death experience, and the findings shouldn't detract from the meaning people have taken from their experiences.
"My work is spiritually neutral," Nelson said, noting that the research can only look at how the brain contributes to near-death experience, and not why the phenomenon occurs.
"The 'why' can't be addressed by scientific inquiry," he said.