Some mammals, such as bears and badgers, spend the winter months in winter sleep. During winter sleep, the animal's body temperature drops a couple of degrees Celsius and its vital functions slow down. Its heart starts beating at a slower rate and its breathing grows slower. The animal does not need any food during its winter sleep.

Animals that sleep through the winter receive energy from their fat supply. They build up this supply by eating a lot of food during the summer and the autumn. The fat layer can take up to one third of the animal's weight when it lies down for its winter sleep. Bears and badgers dig dens to sleep in. They sleep in a curled up position, so that they will stay as warm as they can. An animal's winter sleep is deeper than its normal sleep, but it can still wake up from it, if bothered by people or other animals.

Some mammals, such as hedgehogs and bats, spend their winter in hibernation. Animals hibernate to conserve energy. During hibernation, the mammal's body temperature drops drastically and its vital functions slow down. The energy supply gathered before hibernation is enough to keep the animal alive until the weather warms up again. Hibernation is a deeper state of rest than winter sleep.

All cold-blooded or poikilothermic animals, such as frogs and snakes, spend their winter in a state of brumation or cold hibernation, where their body temperature drops near zero degrees Celsius. During brumation, their vital functions slow down and almost cease completely. Some insects, such as some species of butterfly, also brumate during the winter.

SLEEPING THROUGH THE WINTER

	Winter sleep	Hibernation	Brumation
Body temperature	close to normal	just above freezing	close to zero degrees Celsius
Species	bear, badger	hedgehog, bats	all poikilothermic animals: e.g. reptiles and insects

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