LOSS OF BLOOD FLOW
"One of the most common causes of near-death experiences is fainting," Nelson says. This makes it a good example of when near-death experiences can happen when the person experiencing the NDE is in fact nowhere near death. Researchers have shown that a loss of oxygen flow to the eye will cause tunnel vision. Oxygen deprivation--and even just a sense of fear--can cause the oxygen to stop flowing, and both of those are symptomatic of dying. In Nelson's research, simply fainting was enough to cause several of the effects related to near-death experiences, like a feeling of being out of your own body, or a sense of euphoria.
CHEMICALS RELEASED
A surge of steroids, epinephrine, and adrenaline are released in the body during situations where it's near death, Parnia points out. It could explain the feeling of euphoria, and some of the stranger, hallucinatory effects. It's been suggested that Ketamine, which is released when animals are under attack, could produce similar effects. One of the first theories on near-death experiences, in fact, was that the psychedelic Dimethyltryptamine, or DMT, was released in the brain as soon as it realized it was dying, but that's somewhat speculative: The only way to really test something like that, Nelson says, is to give someone the drug and compare its effects to near-death experiences. Chemicals could very well be be a factor; it's just not an easily proven one, compared to, say, fainting and blood loss.
IT'S A FORM OF REM SLEEP
REM sleep is the type of sleep most closely associated with dreams. Nelson has proposed that, close to death, we enter a type of REM sleep. Put simply, he says, the brain is still functioning enough to realize that it's in danger, despite being asleep. That creates a sort of sleep-state that's spiked with a fight-or-flight response: a form of lucid dreaming, where we're still aware of the situation but are not completely conscious. The sensation of floating around yourself--an out-of-body experience--is consistent with lucid dreamers.
MEMORY IS TRIGGERED
This is only one symptom of near-death experiences, but it's a common one: People report experiencing memories of loved ones or other moments from their past. Research has shown, Nelson says, that our sense of memory kicks in during threatening situations, like being near death. In the more lizard-y parts of our brains, Nelson says, long-term memory and fight or flight are connected, which might be part of the reason people say they remember near-death events so vividly. That phenomenon of near-death experiences might be related, causing the my-life-flashed-before-my-eyes effect or other memories cropping up.
In short: There's not exactly a consensus on what causes them, but the most common effects associated with near-death experiences have been reproduced in some way, and solutions have been offered to their most fundamental causes. But whatever the cause, people coming out of them can feel profoundly changed.