Definition
Hibernation is a body-regulation process in which animals cut their body temperature, heart rate, and metabolism to extreme lows, so they use as little energy as possible to survive a winter when food is scarce and the cold is harsh.
Like Power-Saving Mode, the Body Powers Down
Picture your phone on a freezing day with the battery almost dead. It shuts down background apps and dims the screen to the lowest level so it won't turn off. Animals do something similar. When winter comes, they keep only the bare minimum of functions needed to stay alive and flip the rest of the switches off.
Take a hibernating ground squirrel. Its body temperature normally sits around 37°C (about 99°F). In hibernation, it drops to between 0°C and 5°C (32°F to 41°F), close to the temperature of the soil around it. Its heart, which once beat more than 300 times a minute, slows to just a few beats. It may take a breath only once every several minutes. The body's internal clock runs painfully slowly. The chemical reactions that keep it alive fall to just 2 to 5% of normal, which locks down energy use.
Thanks to this strict power-saving state, animals can get through the long, harsh winter on the fat they packed on during autumn or the food they stored in their dens, using far less energy than usual.
Brown Fat: The Body's Built-In Space Heater
Even when the air around them drops to near freezing, hibernating animals keep their vital organs from freezing. Their secret is a special kind of fat tissue that specializes in making heat. It's called brown fat, or brown adipose tissue.
The white fat you usually think of is a storage room for extra nutrients. Brown fat is more like a personal heater built into the body, burning energy and turning it straight into heat. Its cells are packed with mitochondria, which are designed to release energy right away as heat instead of storing it as a chemical.
Animals use brown fat to carefully guard their body temperature so it never falls below the survival line. And when spring approaches and it's time to wake up, they fire up their brown fat at full blast to raise their body temperature quickly and return to normal.
A Little More Precisely
It's easy to think of hibernation as a long, cozy sleep that lasts for months. But physiologically, hibernation is completely different from the ordinary sleep we get at night.
Daily sleep is an active state of recovery. Brain waves change in regular patterns, memories from the day get sorted, and fatigue fades. Hibernation is different. The brain's electrical signals nearly stop, and the heart, breathing, and cell activity sink to the edge of shutdown. It is a suppressed metabolic state that comes close to suspended animation.
Small animals also briefly warm their bodies back to normal every few days or weeks during hibernation, then drop back into deep hibernation. This is when most of the winter's energy gets used. Why they bother waking up is still debated. Some explanations say they need real sleep, and others say they need to refresh their immune systems.
Common misconceptions
Hibernation is just a very deep sleep that lasts all winter.
Hibernation isn't simple sleep. It's a state close to suspended animation, where metabolism falls to the edge of shutdown. Brain waves and metabolism are heavily suppressed, and small animals periodically warm up before going back into hibernation. There are several hypotheses for why, including a need to recover from sleep.
Where you meet it
The Arctic ground squirrel can keep its body temperature as low as -2.9°C (about 27°F) without its blood freezing, staying in deep hibernation.
Bears lower their body temperature by only about 5 to 6°C, but their metabolic rate falls to around a quarter of normal. It's a unique kind of hibernation, and they can even give birth and nurse their cubs in the middle of it.
In one sentence
Hibernation is a survival strategy. Animals cut body temperature and metabolism to extreme lows to save energy, and use brown fat to keep just enough warmth to make it through the winter.