I experienced this sensation twice last night (and many times in the past). I finally got curious, so I searched the amazing world wide web for answers and explanations. I then found out there's a technical term attached to it. It's called a hypnic jerk. Or if you prefer the more complicated name, it's called a hypnagogic myoclonic twitch.
Turns out, hypnic jerks are incredibly common - around 70% of the total population have these. However, they are more common in people with Restless Legs Syndrome (RLS) or people who are sleep-deprived. And yup, you can count me in. Although... the "restless legs syndrome" doesn't sound so good. I refuse to accept that. hehe I'm only sleep-deprived. ;)
Jake Young, a MD/PhD student said:
The cause of hypnic jerks is not entirely clear. During sleep onset, a part of the brain called the reticular formation sends a signal down the spine that causes your muscles to relax. (Actually, if I understand correctly, it causes a change in response of motor neurons from activation to inhibition. This is called the "reticular response reversal" and has been the subject of considerable research.) The theory is that occasionally this signal to relax is misinterpreted by the muscles as a signal to contract causing the spasm.
Scavenger of blurtit.com said:
The body jerks because it feels like you're falling, and your brain sends out a reflex twitch signal to try to pull you back form the brink. You feel the falling sensation because your brain literally disengages from the body while you're asleep. This prevents you from moving around too much in your sleep, especially if you have very vivid dreams.
Some people don't get a "falling" sensation. It's more like floating for them. In which case, their brains sometimes act to pull them back down to earth. Either way, the lack of contact with the body and where the body is in space is very disorienting for the brain.
Hypnic jerks aren't harmful, although they may wake you up again.
And yup, I wake up every time. =/ But this time, I can smile and say: "Sorry for rocking the bed. I just had a hypnagogic myoclonic twitch..." =)
References: http://scienceblogs.com/ and http://www.blurtit.com/