We camped last week in a 3/4 season tent. It was -27 celcius. I slept in socks, long underwear, a hoodie and a toque. Put a mummy bag inside a rectangle bag and slept through the night. Not much you can do to warm the inside of a tent without propane or something. We had a candle lantern buring. Maybe took the edge off. Definately less than 10 degree difference with outside temp. Probably less than 5 degrees difference.