Some consider this a trick question. That’s because horses actually doze while on their feet and lie down for REM sleep. So, the better question is: Why do horses nap while standing? Mainly because sleeping while lying down can be dangerous. It takes a bit of work for horses to get up, which makes them vulnerable to attacks by predators. To protect themselves, horses instead doze while standing. They’re able to do this through the stay apparatus, a special system of tendons and ligaments that enables a horse to lock the major joints in its legs. The horse can then relax and nap without worrying about falling. When horses need deep sleep, however, they lie down, usually for a series of short intervals that amount to about two to three hours a day. And even then they often have another horse standing nearby and serving as a lookout.