What are endogenous circannual rhythms?

Endogenous circannual rhythms are like internal clocks that help some animals know when to sleep, eat, or go on adventures, all year round.

Imagine you have a toys box that tells you exactly when to play with your favorite toys. Even if no one reminds you, the box knows it’s time for your big dinosaur toy on Saturday and your little car toy on Sunday. That’s kind of how endogenous circannual rhythms work in some animals, they have their own special “toys box” inside them that tells them what to do every season.

Some animals, like bears, don’t need to be told when to hibernate or wake up. Their bodies act like a slow-running clock that ticks all year round, and when the time comes, they know it's time for a long sleep or to run around and eat berries.

Even if the weather is nice in winter or cold in summer, these animals still follow their own rhythm. It’s like having a favorite bedtime even if your room is too bright, you just go to sleep anyway because that's what your body knows to do.