Animals enter hibernation during winter to conserve energy by going into a deep sleep-like state. Mammals, such as gophers, bears, skunks, raccoons, hamsters, and bats, lower their metabolism and enter a state of torpor, but they are not asleep. With a slowed heart rate and lowered body temperature, these animals have adapted to survive cold winters with little or no sustenance.
The dormant state means the hibernating animals minimally eat, drink, move, think, or defecate. Some "deep" hibernators, like bears, almost never rouse themselves once they are secure in their den. Other animals, especially rodents, frequently come out of this state to snack on food harvested during the summer and fall. Cold-blooded creatures, such as reptiles and amphibians, also can be said to hibernate. They are always the same temperature as their surroundings, however, so this "sleep" means something different. For instance, wood frogs actually freeze solid over winter, while a natural antifreeze, glucose sugar, protects their organs.
Once in wintering mode, in a snug den, most animals do not need significant external energy sources. They survive plummeting temperatures by lowering their own body temperature, sometimes to within degrees of the freezing point of water. Physiologically, their bodies reduce their need for energy by almost stopping their heartbeat.
In the months leading up to the cold season, the animal has stored fat by eating more than usual. It receives the little energy it does need by breaking down stored carbohydrates and fats. Organs and muscles even donate some sustenance. A bear actually borrows protein and water from its organs, because it can regenerate them to healthy levels once spring comes.
No one knows exactly what triggers hibernation in various animals. It might be a change in light exposure, measured by melatonin levels, which alerts animals to the coming winter and nudges them to seek out a burrow. A lessening food supply might be what makes them drowsy and lethargic. Biologists have been successful at triggering this state in certain species, like rodents, in the laboratory.
Of course, biologists also use their research to solve human problems. Some scientists think that humans might hibernate one day, such as in order to travel to Mars or lose weight. Researchers are looking for clues to healing people's liver disorders, kidney disease, starvation, or obesity, by studying mammals in this state as well. Humans might carry dormant genes that, when triggered, could regenerate damaged muscles and organs.