For approximately thirty years, I have slept on a two hundred year-old bed that is a nonstandard size, so I had a local company custom make the mattress. I specified, "Make me the hardest mattress you have ever made." They did, and I believe it is impossible to wear this mattress out. Also, I sleep like a baby with zero back pain. I'm convinced hard is best. By the way, I sleep flat on my back with my arms crossed like a vampire.
I remember reading an account of a man who spent years in a Siberian work camp. Mostly, he wrote of all the expected horrors, but he did mention the upside of becoming accustomed to sleeping on a flat board. He maintained that it fixed his bad previous posture and back problems.
Oh, I worked as a traveling rep for years, and I did sleep fairly well at better hotels, but not as well as at home on my old "board" mattress.