Hibernation is a deep sleep that some animals enter to survive winter when food becomes scarce and temperatures drop dangerously low. During hibernation, an animal's body temperature plummets, its heartbeat slows dramatically (sometimes to just a few beats per minute), and it stops eating, drinking, and moving for weeks or even months at a time.

Bears are probably the most famous hibernators, though scientists debate whether bears truly hibernate or enter a lighter sleep state called torpor. Ground squirrels and bats are more extreme hibernators: their body temperature can drop almost to freezing, and they might take only one breath every several minutes. Before hibernating, animals eat enormous amounts of food to build up fat reserves that will sustain them through winter. A chipmunk might stuff its cheeks dozens of times per day in autumn, storing nuts underground while also fattening itself up.

The word hibernate can also describe people who stay home during unpleasant weather. If your family decides to hibernate during a blizzard by staying inside with hot cocoa and board games, you're not actually sleeping for months, you're just avoiding the cold outside. Scientists who study hibernation hope to learn whether humans could someday hibernate during long space voyages to distant planets.