>> The transition in how warm/cold you sleep - could it be related to heat loss from your head? Do you wear any head covering?
Yes I do. In warm weather I usually wear a OR PS50 Watch Cap, cooler a Smart Wool Merino Cap, and cold one of several types of balaclavas. But, next winter I am going to get some sort of down, probably a Nunatek Down Balaclava. I am probably not efficient here, opting for head gear based more on weight than effectiveness.
I also need to do a better job on my feet, which get cold. I usually just sleep in hiking socks. I recently bought a Nunatek Arc Specialist, and Tom talked me into some overfill in the footbox, after discussing my needs and wants. Using this train of thought, I think a pair of down socks or similar in sub freezing temps will help also. To be honest, I probably am not optimizing my winter sleep system. A little more attention to the head and feet are in order.