Koalas almost sleep 20 hours a day. Koalas are a big fan of sleeping. They spend most of their timing in sleeping while the remaining 4 to 5 hours they spend on eating and occasionally mating as well. Sleeping this much longer is very unusual for a herbivore and a marsupial animal.
There is a very simple biological reasoning that explains about the sleeping habits of the koalas i.e., Koalas have to maintain a very slow metabolism rate which helps them to conserve and save their body energy. Therefore; the Koalas slow down their metabolism rate by preferring to lay down static on trees and eventually start sleeping.