Hi, that’s an excellent question! There are really 2 questions here. Even hibernators don’t hibernate all winter. They have to wake up several times during the winter. They may need to wake up so they can pee. Really. Their bodies may also have to do some other important things. They usually stay in their burrows and go right back into hibernation.

Days get shorter in fall. Light and cold tell them when to start hibernating. I did some experiments on this with hamsters that hibernate (they are a bit different from the ones at the pet store). If I made their room colder and made the day shorter (by keeping the lights on for a shorter time), they would hibernate. If I kept the lights on longer and made the room warmer, they would wake up. They would also wake up every few days on their own. I could tell this when I checked on them.

First I would use a tube to blow on them. If they were just sleeping, they would squirm around. If they didn’t move, I would sprinkle oatmeal flakes on their backs. If they still had oatmeal on their backs the next day, I knew they had not moved. If the oatmeal was gone, I knew they had woken up and eaten the oatmeal, then gone back to sleep. They usually didn’t stay in hibernation much longer than a week.

Most hibernators are underground in winter. So how do they know it’s springtime? They may feel the temperature go up. Maybe they can see enough light to tell them that days are getting longer. They might be running out of fat and need to wake up and eat. They also might be getting a signal from the part of their brain that is like a clock. Usually, they start waking up more often near the end of winter. The truth is, no one knows for sure yet. Maybe you will help find the answer one day.

What a fascinating question! For lots of animals, the winter means that is harder to find the food that they need to survive.

One way that animals like bears and chipmunks survive the winter is by hibernating. During this time, the animal’s heart rate and body temperature goes way down. Also, the animal will not need to eat or drink for up to months at a time. In this way, animals can survive winters when food is not around.

Hibernating animals often find a place to hide, like a tunnel under the snow in the case of the polar bear. Because of this, they cannot sense when the days get longer or warmer. So how do these animals know when to wake up?

It turns out that a part of the brain called the hypothalamus has a sort of calendar in it called the circannual rhythm. This is like a stopwatch that gets set when the animal begins to hibernate. Chemical changes occur as time goes on that begin to wake up the animal just in time for warmer weather – and to go eat a lot of food after that long time.

Each animal has an internal ‘alarm clock’ called an internal clock that is controlled by chemical reactions in the brain. Before actual alarm clocks, our internal clocks told us when to go to sleep at night and when to wake up in the morning. Hibernating animals still use their internal clock to wake up from hibernation though! Most hibernating animals sleep through winter when the temperatures are lower and when the outside temperatures start warming back up they will wake up!

Hibernating animals in their dens can’t see light or feel temperature, so how do they know when to wake up? The answer is that they have an internal clock in their brain that times how long they’ve been asleep. When a bear’s body decides it’s time and sounds the alarm, the body starts to warm up and it starts to shiver, which causes it to wake up!