The bag's job is then to insulate enough to keep the heat inside. More clothing would then defeat the purpose of a good insulating bag, relying on your clothing to be the better insulator. Other friends swear by bulking up on the thermals and sweats before hopping in the sack for the night. It makes sense, until it's exposed to the slightest bit of logic. And at that time, it quickly is revealed as having zero grounding in reality.
Insulation is insulation and more insulation equals more insulation.
So the more layers you can put between your tender pink skin and the cold cruel world the warmer you'll be. Your friends who don thermals and sweats before bedding down are on the right track. In particular, cover your head even if your bag has a hood. Your skull is an unbelievably efficient heat radiator, and you want to put a lid on it, so to speak. Keep a candy bar handy should you wake up cold at 4 a.m.