Passage hibernation is a time when animals 'sleep'
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Hibernation is a state of inactivity, in which an animal's heart rate, body temperature, and breathing rate are decreased in order to conserve energy through the cold months of winter. A similar state, known as estivation, occurs in some desert animals during the dry months of summer.
Hibernation is a technique that animals have developed in order to adapt to harsh climates. When food is scarce, an animal may use up more energy maintaining its body temperature and in searching for food than it would receive from consuming the food. Hibernating animals use 70 to 100 times less energy than when active, allowing them to survive until food is once again plentiful.
Many animals sleep more often when food is scarce, but only a few truly hibernate. Hibernation differs from sleep in that a hibernating animal shows a drastic reduction in metabolism, and then awakes relatively slowly. (Metabolism is the process by which cells in an organism break down compounds to produce energy.) By contrast, a sleeping animal decreases its metabolism only slightly, and can wake up almost instantly if disturbed. Also, hibernating animals do not show periods of rapid eye movement (REM), the stage of sleep associated with dreaming in humans.
Bears, which many people think of as the classic hibernating animals, are actually just deep sleepers. They do not significantly lower their metabolism and body temperature. True hibernation occurs only in small mammals, such as bats and woodchucks, and a few birds, such as poorwills and nighthawks. Some species of insects show periods of inactivity during which growth and development cease and metabolism is greatly reduced. This state is generally referred to as diapause, although when correlated with the winter months, it would also fit the definition of hibernation.