I have had mild back pain all my life and often felt stiff getting up in the morning. A while back I moved to a yoga mat (both to be more minimalist and because I thought it would help my back) and missed the padding for a few days, then I was fine. I then started sleeping on the floor with only a sheet under me and it now feels even better. The only two things I feel I need are a normal sized pillow and I do like to use one of those large bamboo pillows that you hold between your legs while on your side.
Right now when I sleep I feel some sort of relief as my back flattens against the hard floor.
For naps or lounging like sitting on a couch reading with a dog, I like them really soft.
What about you guys?
I slept for a few months on just a couple of folded blankets on a hard floor. It was surprisingly comfortable much of the time. My problem now is that because of my shoulder problems I can't stay sleeping on just my back at night anymore. I do back part of the night, then left side, then stomach, then right side then back to back if it's not morning yet. I have a firm foam mattress that works well enough for all 4 positions now.