Grizzly bears instinctively hibernate to survive during the coldest of months, when food is scarce and the weather is harsh. Female grizzlies rely on the hibernation period because they give birth to and care for their newborn cubs during it.
During hibernation, a grizzly's heart rate slows, its need to eliminate wastes stops, and it sleeps. A previously-mated female grizzly won't give birth if she doesn't eat enough prior to hibernation to sustain both herself and her cubs throughout the winter. Bears in captivity don't require hibernation to survive but typically do sleep more and eat less during the coldest months.