DS wears a singlet and a wondersuit or a singlet and fleecy pj's under his 2.5 grobag. Room temp drops to about 18 degrees overnight .
We do the same. We live in a cold house, and I have an oil column heater in DS room, set on the lowest setting, and his room maintains about 18 degrees.

I also put a cotton blanket over the top of him, but I know he kicks that off as he does his cot aerobatics...