Neurologist Prof Kevin Nelson and colleagues have just published a study in the journal Neurology showing that out-of-body experiences and near death experiences are more likely to occur in people who have unusual experiences when falling asleep or waking.
Science Daily reports that:
They found that an out-of-body experience is statistically as likely to occur during a near death experience as it is to occur during the transition between wakefulness and sleep. Nelson suggests that phenomena in the brain’s arousal system, which regulates different states of consciousness including REM sleep and wakefulness, may be the cause for these types of out-of-body displays.
Hallucinations and free-form ideas are very common in the period of entering sleep (called the hypnagogic state) and the period of waking (called the hypnopompic state).
Artists and visionaries throughout history have found inspiration from these unusual sleep-related experiences, as recounted in a recent Fortean Times article.
Link to coverage from Science Daily.
Link to coverage from the Daily Telegraph.
Link to PubMed entry for scientific paper.
Mind Hacks 
Perhaps this is the origin of the widespread folk belief that one’s spirit leaves the body during sleep and must be lured back again…