Why does falling asleep sometimes feel like falling down?

We've covered sleep twitches, the unconscious twitches that people make while they're asleep. They're the result of the unconscious and conscious brain both being online at the same time. Sometimes people twitch themselves awake. This is preferable to another strange sleep tic — the sudden sensation of falling that… » 3/22/12 9:30am 3/22/12 9:30am