When I was in the army, we often slept in tents at -30C temperatures if not colder. I always stripped down to next to nothing, got in my sleeping bag then put my clothes in the sleeping bag so they wouldn't be frozen for the morning.
Wearing clothes in a sleeping bag is the worst thing you can do as you will sweat and eventually freeze.
With a good rating sleeping bag, you should be able to get at the very least 6 hours of a very comfortable sleep and if you wake up because you are cold, get out and shake your sleeping bag to spread the feathers/insulation, it will be good for another 6 hours.
Your body disperses the insulation inside the sleeping bag at contact points and that is when you will feel the cold.
I learned that in basic training a long time ago and I never had a problem sleeping out in the cold.