Hibernation is a deep sleep that animals go through during the winter. Since most animals have a hard time finding food during the winter they go through hibernation to save energy. It saves energy by lowering their metabolism. While hibernating the animal has lower body temperatures. It also slows its heartbeat.
Hibernating animals get ready for winter by storing fat. They also get ready by building their dens in advance. If they do not prepare properly for hibernation they could die from lack of food. They could not be prepared by not building a den to keep warm or not storing enough fat for the winter.