Sleep paralysis.
Woke up in my bed once, and as usual, looked around the room, being lazy, and thinking about going back to sleep. Then I saw some motion out of the corner of my eye. My entire ceiling and all the walls in my room were covered with 3ft across, purple spiders, and more seemed to be rushing in by flattening themselves under the door, before popping back up to be a foot tall. They were swarming at me in order to eat me, and I discovered I couldn't speak or move. Scared the shit out of me, but in the end I went back to sleep as they disappeared.

I've since read that it's the result of becoming conscious before you slip out of the REM cycle, so it's like your dreams get projected onto the world around you. Sounds like it could be cool said like that, but it scared me real bad when it happened.
( Ithyphallophobia, Sat 9 Apr 2011, 3:20, 3 replies)

This happened to me after a night of drinking. I was frozen in place staring at my wall, and I could hear the Tardis sound in my head, followed by the Doctor Who theme music. I kept try to move my head, until I smacked it into the wall.

Still pretty scary.
(Aruu does not put all her trust in root vegetables., Sat 9 Apr 2011, 11:49, closed)

If Wikipedia is to be trusted, this is also the source of the music I hear sometimes as I'm falling asleep. I won't have anything on, and then clear as day I'll hear some Michio Kurihara style guitar music playing from somewhere. That I don't mind, it can actually be quite pleasant, but the visions are always terrifying.
( Ithyphallophobia, Sat 9 Apr 2011, 18:56, closed)

DMT
Your body produces it while you sleep, it's also extremely hallucinogenic, hence the reason dreams are so fucked up.
(Maccy Divided by 0 on, Sun 10 Apr 2011, 16:35, closed)