I wanted to pop in and offer a comment for those who experience the phenomenon of awaking unable to move and experiencing what seem to be hallucinations. This is known as sleep paralysis or hypnagogic experience, and has been linked to anomalous behavior of the temporal lobes. Basically you're awake but the body is not, so you're still in a dream state and unable to move. Google it and read about it.http://en.wikipedia.org/wiki/Sleep_paralysis
Actually per usual Wikepedia has a lot of specious information on it but gives the basic idea.
I know quite a bit about it because I have experienced these very intensely since I was a small child and continue to do so, though not as frequently, to this day. I wake up and see people in the room with me, sometimes not human entities, quite often standing around my bed looking at me. Used to scare the living crap out of me. Then one day I began talking to them and I wasn't scared of them any more.
I don't thing the hypnagogic event has anything to do with meditation artifacts as it occurs during the waking portion of sleep. But if it occurs during meditation it might be very interesting to investigate, as apparently according to brain scans, the temporal lobe is affected by long-term meditation.
Which makes me wonder--speculatively of course--are those entities really imaginary--or does the temporal lobe act as a "flashlight" enabling us to sometimes see the denizens of the other realms?