How and Why Do Horses Sleep Standing Up?
Their legs are locked in the proper position during sleep...

Horses have a unique system of interlocking ligaments and bones in their legs, which serves as a sling to suspend their body weight without strain while their muscles are completely relaxed. Thus, horses don't have to extent any energy consciously to remain standing - their legs are locked in the proper position during sleep.

Most horses do most of their sleeping while standing, but patterns differ. Veterinarians we spoke to said it was not unusual for horses to stand continuously for as long as a month, or more. Because horses are heavy but have relatively fragile bones, lying in one position for a long time can cause muscle cramps.

While one can only speculate about why the horse's body evolved in this fashion, most experts believe that wild horses slept while standing for defensive purposes. Wayne O. Kester, D.V.M., executive director of the American Association of Equine Practitioners, told us that in the wild, the horse's chief means of protection and escape from predators was its speed. "They were much less vulnerable while standing and much less apt to be caught by surprise than when lying down".