Horses sleep standing up for a number of reasons. Their legs can lock in place, enabling them to fall asleep without falling over. Because they are prey animals, horses often do not feel comfortable sleeping on the ground, and most of their sleeping is done during the day rather than at night when the predators are out hunting. Horses have straight backs, so they cannot get up quickly. If a predator were to come while a horse was on the ground, it might not be able to get up fast enough to escape. However, horses do occasionally take short naps laying down during the day, which helps them rest their legs. When horses are in groups, they will often take turns guarding each other as they rest, with one horse standing up near the sleeping horse.