Horses sleep standing up.

The idea of falling asleep while standing up seems quite uncomfortable, especially when there’s a plush couch or bed calling your name. But horses frequently conk out in upright positions that would be far less than ideal for humans. This ability stems from a specialized biological adaptation in horses that allows them to recover vital energy while standing on all four feet.

It’s due to an anatomical feature called the equine stay apparatus, in which the muscles, tendons, and ligaments are all expertly designed to stabilize one another. This apparatus locks a horse’s legs in place without requiring much physical effort. The ability to sleep upright not only helps horses recoup vital energy, but the standing position also allows horses to flee with ease if any predators approach. Despite this unique and impressive ability, horses still need to lie down like the rest of us to enter deeper REM sleep cycles.