That's interesting if it's happened multiple times to others. I wasn't lying down flat on my back and as mentioned was between 45 degrees and upright in a lying or seated position on the bed since I'd just tossed from a bad nightmare. From what I remember I was attempting to pull the the cover upward very quickly and forcefully. I screamed and woke my partner up. I'm unsure if I dropped my head into the punch/strike or what, but it caught me cleanly in the jaw.

So I may have done a 'sit up' into the punch or dropped my head downward into it. The angle I'm not quite as clear on since it was the moment I woke up, but I know my back wasn't completely flat to the bed.