Rapid eye movement sleep. REM sleep occurs in brief spurts of increased activity in the brain and body. REM is considered the dreaming stage of sleep. It is characterized by the darting of the eyes under the eyelids [Source].
Others say that it is, "the stage of sleep that is characterized by decreased muscle tone, rapid eye movements and dreaming," or "the stage of sleep during which the most vivid (though not all) dreams occur. During this stage, the eyes move rapidly, and the activity of the brain's neurons is quite similar to that during waking hours. It is the lightest form of sleep; people awakened during REM usually feel alert and refreshed." Have you ever been aware that you were asleep, but felt you couldn't move your legs or arms, and tried to scream? It's called sleep paralysis and it is due to REM. Your brain zaps your skeletal muscles and paralyses them.
During rapid eye movement sleep, the brain's neurons are just about as excited as they are during waking hours. The resultant sleep is thus light, and that's when we dream.The phase "is marked by extensive physiological changes, such as accelerated respiration, increased brain activity, eye movement, and muscle relaxation." The dreaming is most probably due to the heightened brain activity and the relaxed, or "paralysed," voluntary muscles.
It is said that the brain puts voluntary muscles into this lethargic state to prevent the dreamer acting out their dream. Sleep paralysis occurs when the brain awakes and forgets to rouse voluntary muscles. When sleep paralysis fails to occur (the brain doesn't zap skeletal muscles into paralysis), the person has REM sleep behavior disorder, or RBD; and such people often do act out their dreams.
In the preceding paragraph I said, "the brain awakes and forgets to rouse voluntary muscles." What if it forgets for a good while? The brain's up, aware of what's going on, but the person can't move. Could this constitute an out-of-body experience? Turns out that yes, it could. And well, I'll be damned (no pun intended). First the healing power of prayer was scientifically disproved, and now this. And it seems to explain the whole experience, too, including the tunnel and the white light and the feeling of peacefulness, described by many who've had a near-death-experience.
In the REM study, "researchers compared 55 people who'd had a near-death experience to 55 people of the same age and gender who hadn't had this kind of phenomenon. For this study, a near-death experience was defined as a life-threatening event (such as a heart attack or traffic crash) when a person felt a number of sensations, including a sense of being outside their physical body, unusual alertness, seeing an intense light, and having a feeling of peace" [Source].
For one, people who had had near death experiences were found to have a badly regulated sleep/wakefulness frontier. These people can also have REM while they're awake and ... really, really awake. Segundo, "the same parts of the brain are activated when people dream as in near-death experiences" [Source]. And third, "the near-death study group had a significantly higher rate (60 percent compared with 24 percent)" [Source] of rapid eye movement intrusion.