Bears do not undergo true hibernation, but they do enter a state of lowered activity and alertness during the winter months to conserve energy and survive the coldest part of the year, according to Bear Trust International. During this period of time, bears spend most of their days asleep in their dens or caves, their metabolic and heart rates lower than normal but not low enough to match animals that undergo true hibernation.
A bear in a state of quasi-hibernation can go for more than 100 days without defecating, urinating, or consuming food or water. Animals going through true hibernation must wake regularly to excrete waste and feed on stored food before returning to sleep.
A bear's particular state of rest is referred to as winter sleep. It is differentiated from true hibernation by many factors, one of which is that a hibernating animal is very difficult to awaken from its state of sleep. Bears, by contrast, sleep lightly and can be roused without much difficulty.
Bears depend on their own fat, muscle and organs to provide nourishment to their bodies during their long periods of winter sleep, notes Bear Trust International. The bear's cells break down over time to keep the bear hydrated and nourished even over long periods, and bears can build up their reserves again after winter sleep ends.