How Long Do Sloths Sleep? Myth vs Measured Sleep Time

Where did the 20-hour myth come from?

For decades, sloths were famous as some of the sleepiest animals on Earth. Early studies of sleep relied on captive animals, because recording brain activity in the wild was almost impossible. Captive sloths, with food always nearby and no predators, were recorded sleeping for close to 16 hours a day, and popular accounts pushed that figure up to 18 or even 20 hours.

The idea fit the sloth’s reputation as a lazy animal, so it spread widely. But nobody had checked whether wild sloths behaved the same way.

What happened when scientists measured wild sloths?

In 2008, a team led by Niels Rattenborg of the Max Planck Institute for Ornithology published the first sleep recordings from wild sloths. They fitted wild brown-throated sloths in the rainforest of Panama with a small, lightweight recorder that measured brain waves, a method called electroencephalography, or EEG. The sloths were then released to go about their normal lives in the trees.

The results surprised many people. The wild sloths slept on average about 9.6 hours a day, roughly 6 hours less than the captive figure. The sample was small, only a few animals, but it was the first direct measurement of sloth sleep in nature, and it changed how scientists think about sleep in captive animals in general.

Why do captive sloths sleep more?

Researchers have suggested several reasons why a sloth in a zoo or lab might sleep longer than one in the forest:

• No need to search for food. Captive sloths are given food, while wild sloths must move between trees to find the right leaves.
• No predators. In the wild, sloths have to stay alert for eagles and wild cats.
• No competition or mates to find. Wild sloths may spend waking time on social and breeding activity.
• Boredom. In a simple enclosure there may be little to do except rest.

The wider lesson is that sleep times measured in captivity may not reflect how much an animal really sleeps in nature.

Resting vs sleeping

Part of the confusion comes from the difference between resting and sleeping. Even when awake, a sloth spends a great deal of time sitting or hanging motionless. By some estimates, sloths are still for about 90 percent of the time. To a visitor, a motionless sloth looks asleep, even if it is awake and watching.

Studies of brown-throated sloths describe two resting states: an “awake-alert” state, with the eyes open and blinking, and “behavioral sleep”, with the eyes closed while the animal still hangs in the tree. Only EEG recordings can tell for certain when a sloth is truly asleep.

This stillness is part of their survival strategy. Sloths live on a low-energy diet of leaves, so moving as little as possible saves energy, and not moving also makes them hard for predators to spot. You can read more in why are sloths so slow?

When do sloths sleep?

The answer depends on the type of sloth:

Three-toed sloths are cathemeral, which means they have no strong preference for day or night. They are active in short bouts spread around the clock. Two-toed sloths are mainly nocturnal and usually spend the day resting in a tangle of vines or a leafy fork.

How do sloths sleep in trees without falling?

A sleeping sloth is held in place by its anatomy. Its long, curved claws hook over branches and grip with very little muscle effort, so a sloth can hang for hours without tiring. Many sloths sleep curled into a ball in the fork of a tree, with the head tucked onto the chest and the arms wrapped around the branch. Others hang below the branch by all four feet.

Sleeping high in the canopy also keeps sloths away from many ground predators. Their thick, algae-tinted fur and rounded shape help a sleeping sloth pass for a clump of leaves, which matters because eagles and climbing cats still hunt in the treetops.

How does sloth sleep compare to other animals?

At about 9 to 10 hours a day, a wild sloth sleeps roughly as much as many other mammals of similar size, and somewhat more than an average adult human. Some animals sleep much more: little brown bats, for example, have been recorded sleeping up to about 19–20 hours a day. Others, such as giraffes and elephants, sleep only a few hours. Sloths turn out to be ordinary sleepers that simply spend a lot of their waking time doing very little.