Hibernation is a process of a prolonged period of inactivity that allows animals to survive when food is scarce and the weather is harsh. 
To prepare for hibernation animals will build up a reserve of body fat by eating as much as possible in the lead up to winter, before retreating to somewhere safe where they will enter a torpid state. 
Once settled, almost all of the animal’s bodily functions will either completely stop or slow down significantly, the body temperature will cool, and breathing and heart rate will slow down. 
A bat’s heartrate, for example, will slow to an astonishing 4 beats per minute but can reach 1000 bpm when flying. This reduces the amount of energy the animal’s body must burn to survive.

There are only three UK mammals that truly hibernate: bats, Hazel dormice and European hedgehogs.

Dormice and hedgehogs hibernate right through the winter by developing enough brown body fat (fat reserves they can utilize during prolonged periods of sleep), which they build up by eating fat and sugar-rich foods. In some cases, they can increase their own body weight by over 30%.

As omnivores (an animal which eats both plant and animas) hedgehogs are not picky eaters. These iconic creatures will feast on anything from fruit and insects to beetles and frogs, and will reach a weight of 500-700 grams.
Hedgehogs are also not fussy when deciding where they will overwinter. Some will build their own nest from dead leaves and twigs, whilst others will happily take shelter under a pile of logs, your compost heap or under a garden shed. 
Be careful when moving piles of leaves or wood over the winter as there may be a hibernating hedgehog underneath.

In the woodlands, dormice will gorge on a menu of nuts, seeds and berries, building up vital energy reserves. Dormice construct insulated nests hidden in hedgerows and thickets, where they retreat for hibernation.

During autumn bats are especially active in hunting and finding a place to overwinter. Bats are unable to dig or build their shelters so have to look for the right spot. 
They like to roost in veteran trees that are hollow as well as man-made structures. You might find them wedged into holes in an old brick wall or in the roof of your house, but old barns and abandoned buildings are also favoured spots.