Bears do crawl into a cave and essentially sleep away the winter months, but what they’re doing is not hibernating, at least not in the true sense.
Hibernation is the sleep-like process that involves a depression of metabolic rate, body temperature and breathing rate that many animals enter during the coldest months. It allows animals to survive only on their energy stores (fat and muscle) until the hunting and foraging get good again in the spring. But the list of true or deep hibernators does not include bears, though it does include ground squirrels, hedgehogs, and some butterflies, simply because bears don’t depress their biologic processes enough. Ground squirrels lower their metabolic rates by about 90%, and bears just don't do that. Who knew there were levels of hibernation?