Koalas sleep for a large part of the day because their leafy diet provides little energy.

The longer version

Eucalyptus leaves are fibrous, toxic and low in calories, so a koala must extract energy slowly and spend long hours inactive while fermentation breaks the food down. A large share of the day passes in sleep or quiet rest. The animal also runs a low metabolic rate for its size, which compounds the effect.

Detoxifying the leaves costs energy too, adding to the savings. The habit is not laziness but an energy budget: a diet that delivers little forces a schedule that spends little. Such time as the animal spends awake is largely spent feeding, and even then the movements are slow. Sloths follow comparable logic with their own low-energy food.

Where this entry sits

This collection gathers curious details about sleeping and dreaming, from nightly brain stages and body clocks to naps, yawns and unusual animal habits. It also covers how bedtime customs have changed across centuries, without offering any advice. The full list sits on the Sleep and Dreams page, where every entry is listed in order.