How Do Animals Know When to Come Out of Hibernation?

I feel like winter is coming to an end.  Maybe that is just me and my hopeful thinking, but winter in the Rocky Mountains will be done at some point soon and spring will be upon us.

Springtime is when everything seems to come back to life – trees begin growing new leaves, flowers begin to grow, and animals that have been resting all winter or hibernating begin to wake up and become more active.

I wonder how those mammals in the Rockies that go into hibernation know when it is time to wake up again?  I mean they are simply in a deep sleep and have no way of knowing what is happening in the world?  Yet, somehow their body begins to sense that it is time to wake up.

bear sleeping

A sleeping bear

Hibernating animals have a kind of internal clock that is ticking away while they hibernate.  When they first go into hibernation it is as if the clock gets set.  Then it is ticking away all winter long until it is time for that animal to wake up.

Different animals wake up at different times.  For example, a black bear in the Rocky Mountains and bats in Africa have different lengths of time that they hibernate for.  Animals have developed their own internal clock that lets them know when is the best time for them to wake up – which generally is at a time when there is lots of food available for them to eat.  When they finally wake up they are very hungry, so it is good to wake up when there is lots of good food to eat.

This internal clock that animals have that tells them when to hibernate is similar to the internal clock that tells animals including people when to go asleep and wake up each day.  The daily clock is called our circadian rhythms.  Scientists are still learning more about how a similar process works for hibernating animals, but they call the process circannual rhythms, which is an internal clock that tells the animal when to go into and wake up from hibernation.