Hibernation is an adaptation that helps many animals conserve energy by remaining inactive, greatly slowing their metabolism and reducing their body temperature for days, weeks or even months at a time.
Typically, animals hibernate in order to survive long periods when food is scarce. Hibernating creatures will generally eat a lot of food before hibernation and then survive off the energy stored in their fat.
The physical changes involved in hibernation are far more extreme than what happens during sleep. For some critters, hibernation doesn’t even appear to be restful; indeed, some can emerge exhausted and have to catch up on sleep to recover!