I've often wondered if sleeping on the floor is better for your back.
Because of how poor I grew up, I slept on the floor for most of my life until I went to college and started doing it again after college. Of course sleeping with someone else means sleeping in beds again, but I can generally sleep on a floor just fine. Most people will end up complaining about aches and pains when they do.
I don't have any evidence for or against it, but I personally generally feel better when sleeping on the floor.
IMO thinking about the nonuniform distribution of mass on the body, it becomes apparent that the heavier areas want to sink deeper into a squishy surface than the rest, assuming a uniform firmness distribution of mattress.
So less dense areas of the body are being pressed upward while the rest tries to sink lower, putting all sorts of awkward deformations and strain into the spine, depending on orientation.
For me there's an ideal amount of deformation and that's on the order of a thick rug or yoga mat, just to distribute the load slightly at points of contact (and some thermal insulation). Japanese tatami mats are pretty good since they're basically just slightly compliant breathable floor panels which helps with heat/humidity issues.
As long as my mid section and extremities that don't generally get any propping up on a flat floor can remain unelevated vs. the sinking rest I don't seem to ever have any back problems. One thing that's surprised me is how little thickness is required for it to become an issue. Even sleeping on a birth in my buddy's sailboat without removing the ~2-3" foam cushion can cause me back aches come morning. Remove the cushion and sleep on the underlying bare wood instead? Zero problems, reproduced numerous times over the years.
I hear you re: sleeping with someone else. It's been a sticking point in my past relationships. But for me especially the older I get, I definitely value my physical health, fitness, and lack of chronic pain far more than having a partner to share a bedroom with. It's an easier path to choose without want for children...
Because of how poor I grew up, I slept on the floor for most of my life until I went to college and started doing it again after college. Of course sleeping with someone else means sleeping in beds again, but I can generally sleep on a floor just fine. Most people will end up complaining about aches and pains when they do.
I don't have any evidence for or against it, but I personally generally feel better when sleeping on the floor.