Hibernation is a physiological phenomenon developed by homeothermic animals as an adaptation to winter conditions such as low ambient temperature and the lack of food. Hibernating animals go into a state of induced hypothermia, which allows their metabolism rate to decrease and burn off less energy. Only small mammals (up to 5 kg in weight) are considered to be true hibernators. Aside from hibernation, animals have developed other forms of regulated inactivity, all of which are also characterized by a decrease in metabolism rate. Some fish experience dormancy as reptiles do brumation. Many animals experience aestivation, which is often known as the summer version of hibernation. Before the initiation of hibernation, organisms go through a process of preparation. This preparation consists of acquiring energy storage in the form of food or white and brown adipose tissue. This is followed by a complex physiological process initiates the start of controlled hypothermia and is regulated by the hypothalamus. The occurrence of short periodic arousals, which significantly lower the energy savings of hibernation, is important, but its true purpose has not been identified yet. Every period of controlled hypothermia, including hibernation, always ends in arousal which is mediated by a coordinated sequence of complex physiological events. Research that strives toward the application of hibernation in order to improve human health is currently very popular. It is inspired by events similar to hibernation that have been observed in humans. Aside from medical induction of hibernation in order to lower the intensity of vital organ damage caused by ischemia, scientists are actively working on inducing hibernation in astronauts with the purpose of lowering health risks that occur during sending humans into space.