They may choose burrows, caves, rock crevices, hollow trees, or depressions under fallen trees or brush piles. These locations provide insulation and also protection from hunters and predators. The entrances are just large enough for the bear to squeeze through, opening into a chamber about 2.5 to 5 feet wide and 2 to 3 feet high. For extra insulation, bears sleep on a nest of leaves, grass and other material that they rake into the den. These nests also help keep the cubs off the cold ground.
Hibernation is light.
The heart rate slows to as low as 8 beats per minute. Breathing slows to one breath in 45 seconds. Fertilized eggs now implant in the uterus of female bears that became pregnant in June. Last year's cubs sleep through their first birthday, cozy in the den nest with their warm mother. Hibernation is the way bears conserve energy through the season when their food supply disappears.