Covering your head is the key to warmth. True or false. “You hear claims that you lose 50 to 80 percent of heat through your head,” says Castellani. “If that were true, you could put on a hat and run naked and you’d stay perfectly warm.” It’s true that blood vessels in your head don’t constrict when it gets cold, so you are continually losing heat through it. “But your head comprises only about 8 to 9 percent of the surface area of your body,” he says. Bottom line: “Your head is an extra avenue of heat loss that needs to be taken into account,” Castellani says.