Hibernation is when an animal alternates between torpor (deep sleep) and arousal while holed up in a winter den. The state of torpor is defined as a coma-like state where body temperature, heart rate, and breathing are lowered drastically.
Most hibernators come out of torpor for only a few hours at a time and remain relatively inactive. During arousal, groundhogs usually stay in their burrows except perhaps for an early emergence in February to survey their territories, assess food availability and establish bonds with females. Groundhogs then return to the den to recover and for some more deep sleeping episodes before the final arousal in March.
During hibernation animals are not only incapacitated and vulnerable to an attack from predators, their body is under such stress that temporary brain damage, disorientation and memory loss is commonplace. Also muscles can begin wasting away and, in fat-storing species like groundhogs, the gut undergoes profound atrophy. Most of these negative effects of torpor are reversible but a more thorough examination of hibernation needs to be made before the physiological consequences of this remarkable and widespread phenomenon are understood.
Source: The Role of Energy Availability in Mammalian Hibernation: A Cost-Benefit Approach http://biology.mcgill.ca/faculty/kramer/articles/Humphries%20et%20al_97.pdf