Hibernation

Bears enter their winter hibernation period between October and December. Usually, bears will dig themselves a cave which they will use repeatedly over several years. Naturally occurring caves and crevices may also be used as a winter retreat.

Before winter begins, the bear pads its cave with grass, leaves, ferns, moss and lichen. This provides a comfortable place for dozing, as bears do not actually enter deep hibernation (as, for example, hedgehogs do). Bears do slow their heart and breathing rates, but can be easily awoken and are therefore able to react in case of danger.

Before hibernating, the bear needs to consume enough food to develop a good layer of fat, as bears lose approximately one third of their weight during hibernation.
  

More  Scientific Hibernation