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I've seen a bunch of posts on light and sleep here, but nothing detailed re. mattresses. I ran across this which might prove valuable to future searchers, re. a modular approach to sleep design: https://www.reddit.com/r/Mattress/comments/otdqms/diy_mattresses_an_introductory_guide/
I'd also be curious to hear if others find this to be epistemically sound. Given the value of sleep, having a process for efficiently finding a near ideal sleep surface (or at least one which doesn't cause pain) seems rather important.
My main impression is that there's very little good data about what sleeping surface is optimal.
When I went to a proper sleep doctor, the doctor recommended to me to put my bed on an angle so that my head is higher than my feet. This produced an interesting relaxation effect that I didn't anticipate beforehand.