If you are very relaxed, but persistently unable sleep. But are not alarmed about not being able to sleep anymore, and in a state of almost like not thinking. You may suddenly experience a throbbing sensation in the head/neck, a pulsing in this state.

Normally this enough to trigger alarm in the conscious and fully wake you up. But after this happens many times you can become expectant of it and just let it happen a little longer because you are not immediately alarmed.

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.

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?