The San Diego Zoo has an amazing koala exhibit. It's always the first place I head to, and I could watch those fuzzy little guys all day long. If you're lucky, some of the koalas are awake (most likely eating). If you're REALLY lucky, one or two may even be walking around (in the direction of the food). But most of the time, the koalas are sleeping.
When kids at the exhibit ask why they sleep so much, I usually hear parents say something along the lines of "because they're tired" or "because that's what koalas do", or that the koalas are "high" on eucalyptus oil. Well, those aren't much of an answer (and being drugged on eucalyptus is only a myth), so I thought I'd post the real story.
It isn't surprising that the koalas at the zoo are usually sleeping. Koalas naturally sleep between 18 and 22 hours a day! They have a very slow metabolic rate and need to sleep so much in order to save energy. Koala's main food source is eucalyptus. Eucalyptus leaves are low in nutrition and very high in fiber, which makes them extremely difficult to digest. The koala's slow metabolism allows the animal to retain the leaves in their digestive system for long periods of time in order to extract all the energy they can from them. And since that isn't much in the first place, sleeping in the trees is a method of conserving the energy they do have.
Disclaimer: Don't try this at home! Eucalyptus leaves are extremely poisonous. A koala's digestive system has adapted to host a specific strain of bacteria which detoxifies the chemicals in the eucalyptus leaves that would harm almost all other animals.
Koalas have also evolved a highly adapted skeletal structure that allows them to sit in the tree branches for long periods of time. Their spines are curved to allow them to sit comfortably, and they do not have a tail. Even the koala's rump is covered in very dense fur which provides an extra cushion for sitting.