Head bump

There is a particular sensation that I feel sometimes, and while practicing WBTB and WILD, I have pin-pointed the time when this sensation happens: When REM is coming.

I feel it whether I am lying in bed or standing up, fully awake.

I call it 'head bump' for lack of a better word. It is a feeling of a little bump inside my head, like a black-out which last for a fraction of a second. Almost always it comes as several bumps in a series, with a space of maybe 1/4 of a second between each bump.

This thing is incredibly useful for me to achieve LD:
1) I can use it when doing WILD: it alerts me that the dream is about to start in a few seconds, so I don't lose awareness.
2) If I wake up for some reason, and I feel it, I know that now is an excellent time to go back to sleep, because a dream is just about to start.

I wanted to share this with you guys, in case you find it interesting, and also I'd like to know if anyone here knows what on earth is this thing? I'm certain it is not hypnagogia, but a real physical phenomena ( not a hallucination ), and I'm interested to know what is the physical process that cause it.