The benefits and the risks associated with cold exposure are caused by the stress responses that it triggers, and cold exposure is both a physical and a psychological stressor.[1] Upon exposure to cold water, temperature-sensing ion channels in the skin called transient receptor potential (TRP) channels are activated, which transmit information to the brain through connected sensory neurons.[2] This causes the brain to increase the activation of the sympathetic nervous system (involved in regulating the “fight or flight” response), which triggers numerous physiological responses to deal with the cold.[3]
Immediately after entering uncomfortably cold water, the “cold shock response” is triggered, which causes gasping, hyperventilation, and increased heart rate and blood pressure. The peripheral blood vessels that supply blood to the limbs, hands, and feet also constrict to preserve the core body temperature.[4] As the core temperature begins to drop, skeletal muscles start to shiver, which creates significant amounts of heat through shivering thermogenesis.[5][6] With repeated cold exposure, shivering thermogenesis gradually diminishes, and the body relies more and more on nonshivering thermogenesis for warming.