Phantom vibrations from Apple Watch in the middle of the night

I wear my Apple Watch (series 4, os 7.2) to bed at night; with the Apple Health App putting it into "sleep" do not disturb mode until my preset wake up time. It should not vibrate unless one of my Favorites calls me.

I have only a handfull notifications allowed, and all set to mirror my iPhone.

Yet, I get random/phantom vibrations in the middle of the night. Typically only one buzz - but at 4:03 this morning it was enough to wake me up.

I say phantom because when I checked on both the phone and the watch there is no corresponding "notification". No red dot, nothing in the notification center. No notice of an irregular heart beat/

My watch is NOT set to tell me to breath, or stand, or anything like that. Chimes are off.

I cannot figure out why I get these phantom/random vibrations in the middle of the night, but its become quite annoying.

Any suggestions? Thanks.