Curious Kids: why don't horses sit or lie down even while sleeping?

Curious Kids is a series for children. If you have a question you’d like an expert to answer, send it to curiouskids@theconversation.edu.au You might also like the podcast Imagine This, a co-production between ABC KIDS listen and The Conversation, based on Curious Kids.

Why don’t horses sit or lie down even while sleeping? – A question from Zulfiqar.

I’m glad you have asked about one of the mistakes lots of people make about horses. It’s true they do have an amazing ability to sleep standing up. But they do also sleep lying down. If you’re a horse, you need to be able to do both.

I’ll explain why.

Read more: Curious Kids: how did the months get their names?

Why should horses be able to sleep standing up?

Horses first evolved in open plains. As a prey species (one that other animals eat), they needed to be able to see quickly if another animal that might eat them (a predator) was nearby.

Being able to rest or sleep standing up meant they could get their rest, but if they saw a predator, they could quickly run away. That’s one of the reasons horses run so fast - to get away. The early horses that ran the fastest were more likely to survive.

Three legs on, one leg off

The most interesting part of horses resting standing up is how they do it. In horses there is a special arrangement of muscles and the parts that connect muscles and bones together (ligaments and tendons). This is called the stay apparatus.

The stay apparatus means that horses can stand on three legs and rest the other leg. They can change the leg they rest so all of their legs get a chance to have a break. A horse can weigh more than 500kg so their legs need a rest!

Even though they can sleep standing up, scientists think horses still need to lie down and sleep each day. Your sleep is not the same all night. Everyone goes through different stages of lighter and deeper sleep, and horses are the same.

The deeper stages of sleep are only seen in horses lying down. Both horses and humans need to go through deeper stages of sleep for our brains to work properly.

Read more: Curious Kids: is water blue or is it just reflecting off the sky?

Hello, curious kids! Have you got a question you’d like an expert to answer? Ask an adult to send your question to curiouskids@theconversation.edu.au