Since winter is when food is scarcest, animals that are going to hibernate need to store as much fat as possible before the cold really sets in. Other animals hibernate even when it's warm out and there's food to eat.
In other words, during hibernation, the hibernating animal's metabolism, heart rate, and breath rate slow way down. The animal's body temperature also falls significantly, sometimes below freezing! So the animal won't consume energy heating itself. Some animals even need to periodically “wake up” from hibernation in order to sleep!
