What, you may ask, is a hypnagogic hallucination?
Glad you asked! It's a fairly common hallucination that you get on the edge of sleep, and is responsible for a lot of astral projection crap. Basically, it's the exact opposite of sleepwalking. The chunk of your nervous system that keeps you paralyzed during sleep occasionally fires a little early, or stays fired a little late, so that you're partially awake, but paralyzed. And occasionally--more frequently in some, pretty rarely in most of us, self included--this sleep paralysis is accompanied by extremely vivid hallucinations, as the brain tells the body to move and is overridden by the paralysis, but still tries to provide feedback from the act of moving any way. They tend to be short and banal. So you might have a real-as-reality hallucination of hitting the snooze button, or getting up and leaving the bedroom, as your body fabricates the feedback, only to be disoriented when you wake up a bit more and realize you're still in bed.
For a great many people, these hallucinations can include the sensation of flight--some people prone to them do it for kicks--and thus the out-of-body experience schtick, and probably some of the alien abduction ones, too. I've only managed such a thing once, and while it felt deeply, incredibly real for a good few minutes, it was a classic hallucination--it even started out with panic over being paralyzed. As popular as the whole astral projection schtick is, no one's ever been reliably able to pass the fairly simple test--a friend places an unknown object on top of a bookcase where the only way to see it would be to actually get out of your body and look--and while I don't discount that such things might be possible, unless they can provide proof of that sort, I'm chalking it up to sleep paralysis. (Funny how hostile people get when you ask 'em for proof, though--you'd think if they really believed they were doing it, they'd jump at the chance...)
Anyway, my brief one today was nothing spectacular, being one of the couple-of-seconds, highly mundane variety. I had a nightmare that I was Captain Kirk (god, I'm a loser) on a ship falling into this abyss deep underground, with these huuuuuge canyons, trying to find this monster. When I found it, it was this gigantic human eye with tentacly-spidery legs, the size of a dozen blue-whales duct-taped together, that came leaping across these weird slanted canyons on these giant tentacles and tried to eat my ship. Then it did eat my ship. Anyway, I twitched, thought I woke up, went "Man! I gotta get up and tell James about that!" and started thrashing around to get out from under the blankets muttering "Gotta get up!" to myself. It was as real as real gets--I thrashed a bit, the blankets moved, I got a foot loose, put it on the floor, and it was REAL--and then I woke up a bit more and realized that I hadn't moved at all, and was still completely under the blankets without having shifted so much as an inch. Whole thing lasted only a few seconds, but as is usual, it felt as real as actually getting up would. I can see why so many people believe these experiences are real--they feel real, in ways that something as trivial as LSD doesn't. (Well, maybe at heroic doses it does, but being fond of my brain in it's non-addled state, I never got anywhere near that in my misspent youth.) Brains are cool.
Were I prone to useless speculation, I could go on for hours about whether I REALLY got up that second time, or if this is just a hallucination too, but I have paintings to paint and miles to go before I sleep.