Asleep Standing Up—How Do Horses Do That?

Why don’t horses fall over when they doze off while standing up? The answer lies in the stay apparatus, a complex arrangement of muscles, tendons, and ligaments that stabilizes the front and hind legs while calling for very little muscular energy to maintain a stable posture.

In the horse’s shoulder, a bony groove locks the biceps tendon in place, while other tendons in the upper foreleg hold the shoulder blade and elbow in just the right position for balance.

The horse’s hindquarters are arranged so that the stifle and hock joints are stabilized by a reciprocal mechanism involving tendons and muscles. Neither joint can move unless the other moves also. When a balance point is found, the horse can maintain a standing position while expending minimal energy.

Standing sleep is restful, but horses do lie down for their deepest sleep periods. Dozing horses may appear to be completely unconscious of what is going on around them, but they can rouse almost instantly if a strange or threatening sound or other stimulus is perceived.