Hibernation, or winter sleep, is a state of inactivity and metabolic depression in which some endothermic (warm blooded) animals pass the winter in cold latitudes. It is characterised by low body temperature, slow breathing and heart rate, and a low metabolic rate.
Previous definitions of hibernation commonly referred to a long-term state in which body temperature is significantly decreased, metabolism slows drastically and the animal enters a coma like condition that takes some time to recover from. The problem with this definition is that animals such as bears are excluded because their body temperature drops only slightly and they awake relatively easily. The term hibernation has been redefined, and is now applied based on metabolic suppression rather than on a decline in body temperature.
Often associated with cold temperatures, the purpose of hibernation is to conserve energy during a period when conditions are adverse such as a scarcity of food. Before entering hibernation an animal needs to store enough energy to last the entire winter. Animals will either eat a large amount of food to store as energy in fat deposits, or alternatively catch food to store. To save energy hibernating animals will decrease their metabolic rate, resulting in a decreased body temperature. Hibernation may last several days, weeks, or months depending on the species, ambient temperature, time of year, and individual animal's body condition.
Hibernation during the summer months is known as aestivation (summer sleep). It is a similar condition in which other species pass periods of heat or drought in warm latitudes.
Hibernation and aestivation are believed to have originated in response to a regular and recurrent failure of food supply or of other factors essential to existence due to the seasonal onset of cold or excessively dry hot weather. They allow non-migratory animals to live through unfavourable climatic conditions that which otherwise end in starvation or desiccation were the animals to maintain their normal state of activity.
Hibernation in reptiles is sometimes called brumation. It differs from mammalian hibernation because reptiles are cold-blooded and can't control their own body temperature, so they need to spend the winter in a place that will stay warm enough. Torpor is used as an umbrella term to describe all the various types of temperature- and metabolism-reducing functions. More commonly, it's used to describe short-term periods of reduced temperature that occur as often as every day and only for a few hours at a time.
Just a long nap?
The key difference between hibernation and sleeping is that hibernating animals aren't just sleeping. They undergo drastic physiological changes. The most significant element of hibernation is a drop in body temperature, which can sometimes be as much as 18 C. The vital signs of a hibernating animal are very different from the vital signs of an awake animal. When an animal awakes from hibernation, it exhibits many signs of sleep deprivation and needs time over the next few days to recover.
There are physiological aspects of sleep that are similar to hibernation, such as a reduced heart and breathing rate and lowered body temperature, but these changes are very slight compared to hibernation. Sleep is also pretty easy to break out of; if you are awakened from even deep sleep you can be fully awake within several minutes. Sleep is a mostly mental change and is primarily characterized by changes in brain activity.
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