The thermoregulation is the homeostasis process that maintain the body temperature constant and the human thermoregulatory centre is found in hypothalamus. When the outside temperature decreases, the heat conservation centre in hypothalamus is stimulated generating heat producing responses to effectors such that skeletal muscles and adrenal and thyroid glands.
The response in skeletal muscles is the shivering process in which the muscles contract and relax more than once producing heat by friction.
The response in adrenal and thyroid glands is the secretion of adrenaline and thyroxine that increase the metabolic rate, producing heat.
These responses are generated by the internal corrective mechanism, but if they are not effective, they can be coupled with behavioural mechanisms such that: finding sunny places and putting on extra clothes.