Humans lose heat from those parts of the body which are exposed to the cold environment and head is that part of the body which is most exposed to the elements during the winter. If you'll not wear hats then your brain will direct a number of actions to keep your core temp. steady. By this your blood vessels in your skin gets contracted to preserve heat. By redirecting blood flow your brain keeps your muscles and organs warm. If it is really cold outside then you start shivering which increases heat production in your muscles. This is the only reason why people in cold countries wear hats so that there head is not exposed to the cold environment.