Link to newsround

Bears wake up for spring - but what is hibernation?

A fluffy brown bear sits on the grass next to a tree and looks up at the sky, as if she's enjoying the sunshine.Zoological Society of London

Millie the brown bear at Whipsnade Zoo

Four European brown bears at Whipsnade Zoo have been spotted enjoying the spring sunshine, as they shake off their long winter snooze.

The bears – named Mana, Minnie, Naya and Cinderella – have spent the last 3 months hibernating and are slowly getting used to being up and about again.

But it seems the fluffy foursome can bear-ly keep their eyes open.

Sarah McGregor, a manager at the zoo, said: "They can be quite sluggish when they first wake up, and this year has been no exception - they've definitely wanted to hit the snooze button a few times!"

Why do bears hibernate?

Two fluffy brown bears laying on the grass, whilst another walks next to them.Zoological Society of London

The bears are slowly getting used to being up and about again

Lots of animals hibernate in the winter to help them survive when the weather's cold and food is harder to find.

During hibernation, an animal's heart rate and breathing slows down.

Their body temperature also decreases.

These changes help hibernating animals to survive by using less energy.

A black bear stood amongst some bushes, eating orange berries from one of the branches.Getty Images

Bears eat lots of food to prepare for their long winter rest

According to the Natural History Museum, bears don't fully hibernate like some animals do.

Instead, they go into a lighter sleep called torpor.

During this time, bears use energy from the fat stored in their bodies to stay warm.

Wildlife charity Born Free says this can make them lose up to 40% of their body weight while they rest.

To prepare, bears spend the summer eating as much as they can so they have enough energy for their long winter sleep.