They can’t actually be comfy, can they?

Animals have strange sleep habits; this much is true. But one of the weirdest things about how animals sleep is that some do it standing up. Humans find it hard enough to sleep while sitting in a chair, so how can other animals snooze away happily on their feet?

The answer lies in something called a “stay apparatus.” Let’s take a horse. A horse can lock its legs in a straight standing position without requiring muscles to keep them that way. Why would they develop this type of sleeping habit? It comes down to evolution. Horses that could sleep standing up were better able to run away from predators in the night than those who had to get to their feet before taking off. Interestingly, a horse can’t get Rapid Eye Movement (REM) sleep while standing, so they can’t snooze on their hooves 100 percent of the time.

Horses are joined in their sleep-while-standing club by cows (animals that often choose to lie down to sleep but can do it standing up if they want) and elephants (beasts so large that sleeping for too long on the ground can actually damage their internal organs). Giraffes are also fond of sleeping on their feet for the same reason that horses are—to be able to run away from predators more quickly. And because giraffes are so tall, sleeping on their feet can help them see a predator coming from farther away.

But mammals aren’t the only ones with this unique sleeping position. Flamingos also sleep standing up. This is because they live on caustic salt flats, which means that there aren’t any comfortable places to lie down. And other birds can sleep standing when perched on a tree branch—they use their leg tendons to close their claws tight around the branch and stay that way.