Why does a horse sleep standing up?

We all know that horses sleep on their feet but the how and why is a little more complicated. They sleep upright because they are by nature a prey, an animal that is or was generally hunted and so they always had to be ready to run, flee when danger presented itself. As for the reasons as to how the horse managed to sleep while standing, well those are a little more complicated. The animal’s brain is built to optimise control over the four legs both when still as when in movement. Having four legs, rather than just two, certainly helps to keep the horse upright and well balanced with minimum effort and a horse is able to block his joints, via special ligaments, which also aid his ability to also stay standing when sleeping. A horse also never reaches a state of R.E.M (deep sleep) where the muscles are so relaxed that maintaining balance is impossible.