I am finding expected and unexpected benefits now that I've replaced my mattress with one that is bigger and springier (see sallycrawfordartanddesign.com). Medium density foam on top of springy plastic bases turns out to be my back's perfect sleeping surface.
My back's entire length - including the bit between shoulder blades and neck that I'd almost forgotten existed - feels lighter, warmer, and less stiff. I sit more upright on my cargobike; I turn my head with more ease; I put on my socks in the morning with nimbleness.
Don't put up with an uncomfortable bed a moment longer.