der Winterschlaf
hibernation
A state of inactivity and reduced metabolism that some animals enter during the cold season in order to save energy.
Igel, Fledermäuse und Bären halten Winterschlaf.
Hedgehogs, bats, and bears hibernate.

hibernation
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hibernation
A state of inactivity and reduced metabolism that some animals enter during the cold season in order to save energy.
Igel, Fledermäuse und Bären halten Winterschlaf.
Hedgehogs, bats, and bears hibernate.

The word is a clear compound of 'der Winter' (winter) and 'der Schlaf' (sleep). So it literally describes a sleep that takes place in winter.
Viele Tiere bereiten sich im Herbst auf den Winterschlaf vor.
Many animals prepare for hibernation in the fall.
Common verbs used with 'Winterschlaf' (hibernation) are 'in Winterschlaf fallen' (to fall into hibernation) or 'aus dem Winterschlaf erwachen' (to awaken from hibernation). These expressions can be used both literally and figuratively.
Die Firma erwacht langsam aus ihrem Winterschlaf.
The company is slowly awakening from its hibernation.
The most common and important collocation for animals is 'Winterschlaf halten' (to hibernate). This verb fits the noun perfectly.
Murmeltiere halten bis zu neun Monate Winterschlaf.
Marmots hibernate for up to nine months.
In the figurative sense, 'Winterschlaf' (hibernation) describes a phase of inactivity or standstill. It is often used for clubs, companies, or projects that do nothing for a while.
Nach dem großen Erfolg befindet sich das Projekt nun im Winterschlaf.
After its great success, the project is now in hibernation.
sleep
'der Schlaf' and 'der Winterschlaf' both describe a state of rest and little activity. 'Schlaf' is the normal word for the daily rest of humans and animals. 'Winterschlaf' is more specific and means the long resting state of some animals in winter, during which the body needs very little energy. You can confuse the words because in both cases someone or something is resting. But 'Winterschlaf' does not fit normal sleep at night. In a figurative sense, both can also mean rest or inactivity, but 'Winterschlaf' emphasizes a longer pause, often like in winter.

Im Herbst sucht der Igel einen sicheren Platz für den Winterschlaf.
In autumn, the hedgehog looks for a safe place for hibernation.
Nach der langen Reise brauchte Lisa viel Schlaf.
After the long trip, Lisa needed a lot of sleep.