Side note: I thought my mattress was giving me lots of back problems (and incidentally sleep problems), and replaced it, and everything was fixed. Some months later I resumed sleeping with a certain very comfortable, too-small blanket. I hadn't realized I had stopped when I switched mattresses. The back problems returned. I got rid of the blanket, they went away. The too-small blanket had caused me to curl up into too tight a ball while sleeping.