This is actually a very common thing, which happens to me each and every single night as I fall asleep. I'm not so sure "that your body thinks its dying" but it's typical unconscious sort of... check, to see if you're still asleep. It's actually completely related to the phenomenon of sleep paralysis. I believe that this is because when we go into sleep paralysis it is so that we do not physically act out our dreams... these little jerks or sensations are minute signals sent out from the brain into the body to test whether or not we have fallen asleep yet. This is also why if we lie still long enough we get the urge to itch, or roll over. Both of these are similar test signals to see how the body/mind connection is. As in, if you're awake you will most likely roll over, scratch that itch, or move after the jerk.

Theoretically if you consciously refuse these sensations, the mind will think the body is asleep (or does the body think the mind's asleep?) The muscle spasms are common, though may vary in degree, are just used to gauge how 'asleep' we are. When a muscle spasms the brain sends out a signal to that area because it is still 'awake' and hence spasms, so the signal is essentially dispatching more sleep paralysis to the awake area.

Though this is a good thing, there's a way to make this work for you because it's just a battle of wills between the mind and the body. You can also FAKE a muscle spasm by twitching or clenching a muscle in your arm, leg, face, finger etc... in order to induce sleep paralysis sooner.