Is this the sinking, tremor feeling you get after your awake state, but before the sleep state? That weird limbo where, if your brain jolts back on from this state, it's as if you awoke from a dream - even though you only dozed off for a couple of seconds?

I have experienced something that sounds like this, but during the first few minutes of falling asleep after a long time without.

Sometimes I woried that I was going to "pop a vessel", but after a couple of times, didn't worry so much.

Is it the same thing as you mean? I never experienced it "spontaneously", only while settling down to sleep before drifting off.