The human body heats up from external sources like clothing and environment, and internal sources like metabolic heat generation.

“We heat up from the outside, meaning the things that we come into contact with, the clothing that we put on our body, whether or not there's heat in the room, or whether or not it's cold outside or cold in a room, and we heat up from the inside. Our body has the capacity to generate more heat or to cool down, meaning to turn off the heating process, and it can do that in ways that match the external environment.”