Throw away the bed! The bed is too old, the mattress is sunken or the bed is sunken. The surface of the bed is uneven. There is too much draft in that cold bedroom.
Throw away the bed, move out of the cold bedroom and place the bed against an inside wall in a corner of a room that has a small door and small windows or no windows.
Make sure the bed is placed in a corner of a room that is known to be warmer than the rest of the house. Then take a hot shower every night before going to bed and another hot shower every morning after getting out of bed.
I actually prefer sleeping on three or four rugs stacked one on top of the other. Three or four cheap rugs stacked on top of each other actually make a rather soft bed.
Sleep on the floor.
Muscle tightening during sleep is often a reaction to the daytime posture one is forced into by an “ergonomic” chair. Check the posture while sitting in an “ergonomic” office chair. These chairs are actually “ergonomically harmful” to one’s “ergonomic health.” In this case, the back muscles are being cramped by the “ergonomic” chair and during sleep, the back muscles are trying to straighten themselves out or stretch themselves out, i.e., they feel as if they are tightening up.