But there may be another factor at work. When I sleep on the front bedroom's mattress, my knees scream at me when I try to stand on them and my hips twinge as I roll over. *Possibly* I just had a very good massage on Sunday, but that hasn't happened for the last three or four nights. I can sleep happily without a pillow between my knees, or under them if I'm on my back. Though I have nothing to hold on to when I stand up, my knees don't buckle under me as they do when I pull myself up on the door knob in the front room. My elbows don't twinge even after all the side sleeping I do. It's not that I'm usually happier on a futon than on a mattress: my thrashing sleep style is impeded by the futon's inertness, which is why I started sleeping on the mattress in the first place. I just hurt less now.
And the clincher is this: none of my pillows are the right height when I sleep on the mattress, while all my pillows are just fine on the futon. The solution, which I hope is the right one, is to buy a new firm mattress. That won't let me sink down as I do in the futon, enfolded and supported by all that REALLY HEAVY cotton stuffing. (Well, and foam core: quite a lot of it, IIRC.) But maybe I can trade supported for enfolded, and still roll over as is my sleeptime habit. Worth a try.