Hibernation is a seasonal period of extremely low metabolic activity that permits certain animals to survive winter, when little or no food is available. Most hibernators accumulate large amounts of fat over the summer and fall to use as the primary fuel source during the hibernation season. By also reducing energy expenditure and living below ground in burrows, they can survive winter when there is heavy snowfall and air temperature hovers near freezing. Hibernation is, however, only one type of winter dormancy.

A season of relative inactivity occurs in a vast array of plants and animals. A type of winter dormancy occurs in several species of insects, amphibians, reptiles, birds, and mammals. Only one avian species is known to hibernate, although other species become hypometabolic for several hours each day during the winter. It is generally agreed that the primary function of winter dormancy, and hence hibernation, is to conserve energy during an energetically challenging time of year when little or no food is available. Dormant ectotherms remain hypometabolic throughout the entire winter and become active again in the spring when high ambient temperatures return. In contrast, in mammals a season of hibernation is characterized by prolonged intervals (days or weeks) of hypometabolism that alternate with brief intervals of high metabolism (less than 24 h) over the course of 5–6 months.