Do you have a memory foam mattress? Memory foam is known for getting hard and inflexible at low temperatures.
However, memory foam mattresses are not memory foam all the way through. They are a layer of memory foam bonded to regular foam underneath. You might try flipping your mattress over so the memory foam is on the bottom and you are sleeping on the regular foam and see if that helps.
I don't like to make plans. They cause the word "PREMEDITATED" to get used in court!
DON'T FEED THE VULTURES!
My Body is a Temple! Ancient, Crumbling, Probably Cursed . . .