From my experience, sleeping on a bare cot and sleeping on the bare ground are the same -- they both suck heat out of you.

Either place, put down a piece of Reflectix flexible insulation (that mylar/plastic laminated stuff from the hardware store), and then a closed-cell foam pad on top of that, and you'll be fine.

And if you're sitting around a campfire in a folding chair and your back is freezing, grab that Reflectix, fold it in half, and put it under/behind you in the chair. Toasty.

Sue