A bizarre, dream-like paralysis experienced when people wake from a deep sleep can cause some sufferers to believe they have seen a ghost or even to have been abducted by aliens, despite being otherwise mentally normal.
Don't let the rather dry article fool you—it can be downright scary when these things happen.
Years ago, this was possibly in 1993 or 1994, I had fallen asleep for a nap in the late afternoon. Around 7:00 or 7:30 pm in the evening I awoke, but couldn't move. I felt as if I had been drugged; I was mentally alert, but I could not move to save my life; I could barely get my eyes open and only with a huge effort on my part.
While this was going on, I could feel this … presence. It had unlocked the front door, made its way through the condo towards my room. I was struggling to move. Heck, I was struggling to get my eyes open!
It was a horrible feeling.
The presence made its way to the bedroom door and was in the process of going through it when I was able to snap out of the paralysis, upon which the presence vanished.
I was quite shaken afterwards and didn't go back to sleep for quite some time as I was calming down from the incident.
A few weeks later I related the experience to a friend of mine and he was convinced that it was a malevolent spirit (I think he used the phrase “demon from Hell”) that attempted to possess me. I suppose it could have been worse—it could have been an attempted alien abduction.