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Even though I've tried once or twice, I've never experienced it personally, but I know people who have. One said it happened unexpectedly - she suddenly found herself up near the ceiling in the corner of the room looking down on her body lying in bed. Her first reaction was one of excitement and amazement, but then she said she panicked because it occurred to her that she might not be able to re-enter her body - and instantly she was back. I asked her the obvious question - whether she might have been half asleep at the time - but she said definitely not - she'd only just gone to bed - and it was real. She said she wouldn't want it to happen again because after her initial reaction of excitement, it scared her.
Another can do it at will, and says he can decide where his 'spirit' will go when he's experiencing this. He thinks it's incredible - and the last time he 'travelled' he says he saw space from space! Sounds pretty fantastic, but they're both level headed people and I've no reason to disbelieve either of them.
It does seem to be similar to Near Death Experience, in that the 'soul' leaves the body and returns, but I agree with wizard that it's not a religious experience. Some of these experiences are no doubt psychological, but to conclude that they're all psychological would be taking rather a narrow view. There's so much we don't understand about the human mind, the soul, the universe and about nature - and I think this has more to do with the laws of nature than anything else. It would be marvellous if some day science could explain it.