My issue, at least to the extent I understand it, is that a lot of the tendons and ligaments of my lower spine are overstretched. There's not a lot to be done about it, so the plan is muscular strength and mindful posture. If, e.g., I sit without a lumbar support, or if I do too much spinal flexion, I'll get woken up in the middle of the night with leg cramps due to nerve pressure. If it's really bad, it'll happen during the day. Fortunately, I know what I need to do and I do it, but part of that for me is careful sleeping posture, because the other side of the coin is that my lumbar is also arthritic, and if I get myself too far into extension for too long, I don't have disc pressure but I'm stiff as a board, e.g., if I bench press and don't maximally glute flex at the same time, when I get up from the bench, I can barely bend over enough to touch my knees.
Worth noting, in giving my little history here, that I was in no hurry after my back injury to start stretching my lower back. I waited until the 9 year mark, give or take, before I did any lower back stretching - before that, it was all strengthening, and a lot of being really stiff in my lumbar but being happy that I was relieving the nerve pressure. (Back injury's 20th anniversary will be this Fall, and there's family history, too - my father wore a back brace, basically a big back cast, for a while as a teenager to deal with back issues although I never completely understood what his issues were.)
I'm not complaining, just explaining my particulars - having a mattress (where this discussion started) that's the right combination of soft and hard makes a difference to me. That, plus a firm pillow (I use an old sofa cushion in a pillow case) for under my knees when I'm sleeping on my back or between my knees when I'm sleeping on my side, plus a very thin pillow under my head, and I'm good to go. I still sleep a lot on my back, something I never did before my back injury, because after it happened, being on my back with a big pillow under my knees was the only position I could be in, period, for quite a while.
Remind me not to spell all that out in writing too often - pretty frightening to look at it.
-S-