Side or back usually. I dunno I've camped a lot in my life and slept in odd places and always had sort of shitty beds. I think i've just gotten used to it over the years.
I still prefer to have something underneath me. You're right sleeping directly on the floor sucks. I meant more on the floor with a blanket or some foam or something underneath me as opposed to a soft sinking type bed.
The floor would make me numb.