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Hibernation is a time of inactivity. Some endothermic (warm-blooded) animals hibernate, usually during the winter, when food is short. They fall into a sleep-like state. They regulate their metabolism to consume less energy. They lower their body temperature, slow their breathing, and slow other vital functions. During hibernation, the body uses fat for energy, which the animal has gathered in summer and autumn. They hibernate from the end of autumn to the start of winter. Animals try and eat as much as they can to increase the fats in their body.
The phases of sleeping (called torpor) are different for the different animals. Hedgehogs sleep for 1–3 weeks, without waking, The fat dormouse sleeps for 20–33 days, without waking. The winter sleep of bears is not as deep as hibernation.
Only mammals hibernate. However, a few birds including hummingbirds do something similar.