REM Intrusion is a condition where certain characteristics of Rapid Eye Movement (REM) sleep occurs while awake. It is not the same as daydreaming or 'dreaming while awake', but rather a situation where certain areas of the brain become over-active.
Symptoms include a sensation of awakening, yet being unable to control the limbs, whilst experiencing realistic visual and aural stimuli that don't actually exit (hypnagogic hallucinations), suddenly entering a state of sleep-like relaxation (cataplexy), and sleep paralysis.
REM intrusion has been linked to several different afflictions, but it transpires that most people have experienced it at some point in their lives. A lesser form of REM intrusion may be experienced if you suddenly wake up in the middle of the night, without the natural transition from sleep to wakenness, and some people argue that REM intrusion may be part of the reason for why most people occasionally experience muscle spasms just before they fall asleep.
People suffering from sleep-related illnesses such as narcolepsy, insomnia and similar are more likely to experience REM intrusion on a more frequent basis.
A recent study by the University of Kentucky has drawn parallels between REM Intrusion and 'near death experiences', and the current theory stands that what people experience as euphoria, scenes of bright light, etc, might be due to REM intrusion brought on by the onset of brain death. Some physicians believe it may be the last blip of the brain shutting itself down to a state of low-oxygen-consumption, which could possibly be a mechanism to keep the brain alive for a short yet crucial extra amount of time. People who have experienced a 'near death experience' appear to be more likely to have experienced REM intrusion in the past, and are more likely to experience REM intrusion than control groups.
Sources: June 2000 issue of the Scottish Medical Journal, and issue 2573 of New Scientist.