Although the intense shivering that is induced by unaccustomed cold exposure can cause significant increases in the metabolic rate, shivering isn’t necessary to get a metabolic boost from cold exposure. In one study, participants who were exposed to mild cold (66°F/19°C) for 10 hours every night for a month had a 42% increase in brown fat volume and a 10% increase in fat metabolic activity.[1] Although the participants in that particular study resided in a clinical research unit for the duration of the study, which allowed the researchers to test a specific set of conditions that may not reflect those of free-living humans, it is clear that humans acclimate to cold temperatures in part through increased brown adipose tissue activity and nonshivering thermogenesis.
Additionally, with regular cold exposure, shivering thermogenesis (ST) attenuates (decreases over time with exposure) and nonshivering thermogenesis (NST) increases.[2] The transition from ST to NST can occur relatively fast, as noted in the above study, in which daily cold water immersion for 1 hour in 14°C (57°F) water reduced shivering thermogenesis by 20% without a change in total heat production.[2]