Why and in what ways do horses sleep standing up?

Many people have noticed that it is extremely rare to catch a horse sleeping, unless you are a groom and are constantly with the animal. When a horse does decide to fall into the arms of Morpheus, it remains standing. Why does the animal do this?

This is due to nature’s survival goal: the horse must balance its need to sleep with the constant threats from predators. If you doze standing up, you can orient and retreat much faster in the event of an attack.