When people ask where do grizzly bears sleep, the response is often that they hibernate all winter long. This is not completely accurate, but it is still a fascinating fact to consider: grizzly bears (Ursus arctos horribilis) can go into a state of deep sleep lasting months at a time without losing too much energy. This restful state is known by many different names: winter sleep, winter slumber, torpor and hibernation. Its purpose is to conserve food in a season of sparse resources.
During their seasonal slumber, a grizzly's body temperature falls to just above freezing. Its heart rate and breathing are greatly reduced, and it is unable to be woken up. But a grizzly is still able to rouse from its slumber if it needs to forage or defend itself. This ability is one of the reasons that grizzlies can survive hibernation and forage all through the winter.
Most grizzly dens are made from branches, spruce boughs, duff and other material lining a natural cave or ground crevice. Ideally, dens are found on north-facing slopes since these provide the best thermal insulation. The den entrance is usually dug at a downward angle to minimize heat loss and the chamber is built just large enough to allow for easy breathing. Males and females with cubs typically dig larger dens.
As spring approaches, a grizzly's appetite returns and it wakes from its long slumber to forage. It may even forgo hibernation and enter a torpor state during the summer to save its remaining fat stores.