In reply to Frogger:
I can't say for sure, but can contribute an anecdote with accurately measured temperatures:
Camping in Norway in January, the outside temperature was -18C when we retired. With 3 people in the tent the temperature very quickly (~30 mins) reached -5C 'in the middle', obviously a little colder around the edges. Lesson: having people in a tent makes a huge difference to the air temperature. Having a good mat (or a few) is more important than the sleeping bag.
Also remember it's important to ventilate the tent when it's cold, or you will get a large build up of frost on the inner and your sleeping bags.