I won't advise this for everyone. Everybody's anatomy is different.
I once had a super hard mattress & it slowly made me develop back problems. It took me a while to figure out.
For me a hard surface makes my muscles tense up & my spine is not flat. My buttocks is constantly under tension. If you move around at night alot it means that the mattress is not good for you. The perfect mattress for me is when there's no gap between mattress & my body. I also sleep on my sides so a hard surface will be bad for my hips
It does take time for the body to adjust to the floor, and doing stretches every morning helps relive the tension over time, a good routine and habit. A Hard mattress vs floor are quite different, I can say that it takes a lot of time to adjust both the mind and body, once that period is over (like breaking in a new bike
), it's hard to sleep on anything else.
I also mentioned the Japanese Futon to the OP, they are cotton batted and some over 5-6" Thick, and can last a lifetime if well cared for, and not as hard as directly on the floor.
A curved back is a good thing, like an arch on a building, the load is transferred to the rear shoulder blades and down toward the illiac bones. Supporting the back with a mattress, provides an upward force actually pushing the spinal column, and leaving the primary shoulder blades and illiac bones suspended. Picture a suspension bridge, now with the road pushed from below and the columns sitting in sand. The spinal column sends signals and fluids to rebuild the body while we sleep, mattresses slow down these fluids.
You are right, everyone has a different anatomy, that is largely a result of our environment, diet, and lifestyle. All of which greatly differ from individual to individual, sleep is more structural, and we all have the same bones. A good nights rest should support the primary bones in the body transferring the weight into a solid surface fluidly, which in turn transfers it to the ground/earth like any structure. Think of the primary contact points, head/neck. shoulder blades, hip bones, knees (supported by
bolster to create an arch), and the feet. The weight of the body should pass from all 9 points into the "mattress" properly. All 9 points/areas of contact will harden over time when you sleep on the floor/hard surface, the bones get stronger like concrete over time.
Tossing and turning at night, could be other factors such as
melatonin levels rather than the mattress, something mattress companies don't want you to figure out. Once again, that's related to environment, lifestyle and diet.
I'm just throwing points out not saying anyone choice or method of sleep is wrong/right, end of the day, everyone just wants a goods night rest