Torpor is a kind of deep sleep accompanied by drastically lowered body temperature, heart rate, and breathing. Like a short-term hibernation, this state is common to a variety of burrowing animals, some birds, and many animals that live in cold climates.
Animals can go into a state of torpor, usually for relatively short periods during the day or night, to conserve energy. In a cold climate, keeping a high body temperature requires a great deal of energy. By going into a deep sleep, animals like ground squirrels, bears, and badgers can conserve energy. The state lowers the animal's natural body temperature and slows breathing and heart rate, so the animal uses less energy.
Small animals, or those with limited food supply, benefit the most from torpor. These creatures, as well as birds, have a limited ability to store body fat. Birds need to stay light to fly, and small animals simply don't have very much room. Because these animals can't store as much energy, it makes sense for them to enter a state of torpor when they are not feeding. This way, they conserve energy for when they can get food.
Animals who have limited windows of opportunity to feed also find torpor valuable. If an animal can only hunt at night, and cannot find food during the day, it has little reason to remain active in daylight hours. By going into a deep sleep during times of inactivity, such animals save even more energy.
Torpor has disadvantages as well as advantages. Animals in this state are very sluggish, and may be almost completely unaware of their surroundings. They cannot just wake up and become active instantly, but they take a while to bring their body temperature, heart rate, and breathing back to normal levels. If an animal is attacked while in torpor, it has very little ability to defend itself or get away. Animals entering this state look for a safe place, either a hidden nest or burrow, so that they will be relatively secure while in their defenseless state.
Hibernation and estivation, sleeping during the summer, can be though of as extended states of torpor. Some people argue that the term should be specifically used to refer to shorter sleep times, but the changes to the animal's body and the goal of conserving energy during times of inactivity are the same.