Why do some people "jump" when falling asleep? --- Sharon, Lakeland, Fla.
The funny thing is, no one really knows yet. The phenomenon does have a name - it's called a Hypnic Jerk - and there are several well-known theories, but all that science has to offer right now is speculation. This page offers the standard explanations.
I experience a hypnic jerk a couple times a year. For me it always happens like this. I am really tired. I lie down and start to fall asleep almost immediately. I enter a weird dream state very quickly. Something starlting happens in the dream and all my muscles convulse in a giant twitch in reaction to the dream. The convulsion wakes me up, so I remember it. Then I fall right back asleep because I am really tired. Usually it doesn't happen again.
Which leads me to Marshall's theory of hypnic jerks. In normal REM sleep, there is a mechanism that shuts off your muscles (the result is called muscle atonia). This is why humans generally don't flail around in REM sleep like dogs sometimes do. Our eyes twitch, maybe our lips, and sometimes the breathing is affected, but that's about it. All the other muscles are disabled. My theory is that a hypnic jerk happens because the dream is generating a strong response (all the muscles in the body need to move at once) and the whole muscle atonia thing hasn't had time to take full effect yet because sleep and dreaming happened almost immediately after lying down.
Under this theory, hypnic jerk is fairly rare for most people because being that tired is rare, and because you need to have the right kind of dream to get the full-body convulsion. Also, since the convulsion wakes you up, you remember both the dream and the convulsion, unlike most dreams which you forget.
If you have your own theory, feel free to put it in the comments.