What is hibernation in mammals?

Hibernation is a survival strategy in which a mammal dramatically reduces metabolism, heart rate, breathing, and body temperature. Animals such as ground squirrels, bats, and hedgehogs can enter this state for long periods, though not all “winter sleepers” use it in exactly the same way. Some species are true hibernators, while others enter a lighter dormancy called torpor. Brown bears are often discussed with hibernation, but their winter sleep is physiologically different from the deep hibernation seen in small mammals.