Different animal species hibernate at different times, and each species has a different way of knowing when the time is right. Hibernation is most closely regulated by temperature. When it gets cold outside, animals get ready to hibernate. When it warms up, they wake up. Therefore, hibernation periods can vary depending on the weather that year. An Indian summer and an early thaw could result in a very short hibernation.
Some species keep a close eye on their food supplies. When they dwindle, the animal knows it's time to gather up whatever is left and turn in for the winter. Photoperiod (the length of the day) triggers hibernation for others.
Even if an animal has no idea what the outside temperature is, how early the sun is setting or the current state of food supplies, many would still enter a hibernation state around the same time each year. Experiments under these conditions have proven that some species will automatically enter hibernation at the appropriate time, guided by an internal biological "calendar" [source: Roots]. These circannual rhythms aren't fully understood, but all animals are affected by them, even humans. Animals that go into daily torpor depend instead on circadian rhythms, the daily version.
Preparation is required to hibernate successfully. Some animals prepare a den (also known as a hibernacula) and line it with insulating material, just as leaves or mud. Ground squirrels and lemurs do this. Polar bears dig tunnels in the snow. Other bears might spend the winter in a hollow beside a tree or a shallow cave, leaving them partly exposed to the weather. Bats are well-known for wintering in caves or attics.
Next comes food storage. Food can be kept in the den if it's nonperishable, but this requires the animal to wake up briefly during the winter to eat. Another option is to eat a large amount of food starting in late summer, building up a reserve of internal fat. Some animals even do both. If enough food can't be found to prepare for hibernation, it can be delayed.