This is that falling sensation and sudden jolt you sometimes feel just as you're falling asleep. Scientifically named hypnagogic myoclonic twitch, it's also the third leading cause for embarrassment on sleepovers, behind bed-wetting and sleep farting. The muscle spasm may be light enough to be misinterpreted as a dream or it can be so jarring it physically shakes you awake.
It's fairly common (studies say around 70 percent of you have experienced it). The likelihood is increased if a person is exhausted or is sleeping in an uncomfortable position. That means the working poor and the homeless are probably more likely to experience a hypnic jerk, though scientists would probably rather not bother asking them. As far as recognized members of society, the muscle twitch is a normal and well-documented event.
What the hell causes it?
Like any good freakish event, the experts can't agree on what causes it. Most researchers feel that when muscles begin to slack while preparing for sleep the brain senses and misinterprets the relaxation signals, and assumes the body is falling. A little presumptuous on the brain's part to go around "assuming" you're falling while lying down. Then again it's nice to know the most important organ is on the lookout for trapped doors with mattresses on top.
If you want to experience this feeling at your desk just lean back in your chair, a little past the point of no return. Once gravity reminds you who's boss your body enters that frenzied states of "lost-balance/I hope that girl I like isn't watching." The sensations are so physically comparable that the brain analyzes them the same.
So there. Don't you feel better now?