They can’t help it – they just don’t have the energy to do anything
We’ve all had one of those day where it’s a huge challenge just to keep your eyes open – that’s daily life for a koala. If you come across one of these tree-dwelling marsupials there’s a good chance it’ll be asleep, as they spend just four to six hours a night awake and snooze on a branch for the rest of the day.
Koalas live primarily on eucalyptus, and there’s a widespread myth that a chemical in the leaves makes the koalas drunk or stoned and it’s this that makes them so sleepy. Despite the popularity of this idea, it’s just not true. Eucalyptus is poisonous to many animals, but the koala’s digestive system has evolved to cope with the toxins.
They might not be affected by its toxic qualities, but koalas still digest eucalyptus very slowly. It’s highly fibrous and has very low nutritional value, so all that sleep is needed to conserve their limited energy and process the leaves.
Koalas have another energy-saving tactic; their brains are tiny. They’re costly organs, but the koala has one of the smallest brains of any mammal relative to its body size. It’s also smooth, giving it a smaller surface area than the folded brains of other mammals.