I can only guess but i have also heard the head and feet thing are myths. ISot may be myths but i still put on fresh wool socks and a stocking cap when I go to the bag in cold weather. So my guess is the main Trunk.
I don't know if it's true but I figure that if you lose 60% of your body heat, you're dead. So I have the good sense to wear thick socks, insulated boots, thermal underwear, layered clothing, gloves and a warm hat.
Well, maybe the percentage is off but you can lose a significant amount of heat and moisture if your head is not covered. At any rate it is an ill-advised move to be in a hostile environment without head cover.