I second what Tuvok says, we have been married nearly 50 years and due to my aversion to sprung mattresses since a teenager dictated the choice. People using any of our beds have been known to ask why they sleep so well. Some 20 years or so ago we changed all of them to a very firm foam (not memory - I am certain I would not like a non-flat bed) called Frostex by some. It is a composite foam of, in effect, recycled foam but not really. The foam is around 70mm thick on a flat board (sheet material) and a couple of them are only 4cm thick. One excellent bonus is that they remain unchanged for decades and are cheap to begin with.