Again, I will mention that it depends on what you think is cold.
We try and go out for 1 last weekend in October before I need to winterize. Nights could be about 3-4 C, 35-38F with the potential for frost. I run 2 electric heaters and set the furnace at about 65F. The furnace might cycle a few times. Of course to keep condensation manageable, we crack some windows and leave the tent ends unzipped a little. Use PUGS and Reflectix in the tent ends. When we go to bed, the sleeping bag does feel cold, but it warms up quick. We use a queen size 10C rated sleeping bag. Maybe a mohair blanket on top. Usually by morning I feel hot as the hybrid doesn't really get that cold. Suggest putting some carpet on the floor to keep your feet from feeling the cold from underneath.
Then again, my son is in Scouts and they have winter camped to -10C in a tent. Nice mummy bag and sleeping on a thermarest ground pad, sleeping toque and warm pj's and all is good.
Just need the right equipment for the right job.