Equids -- specifically horses, donkeys and zebras -- are the only animals that sleep standing up, an evolutionary talent that allows them to doze and still be ready to flee from predators at a moment’s notice. Mature horses, for example, frequently rest standing up, aided by what’s known as the “stay apparatus” of the forelegs and the “check apparatus” of the hind legs. This anatomical system lets them snooze without falling down, with little or no muscular effort.
Of course, horses and other equids also lie down to get deeper REM sleep, and may even snore and twitch as though they are dreaming.
The sleeping habits of horses:
I remember my childhood days. We had one horse and horse-cart which my grandfather used to visit the Eye Hospital daily. We were spread grass bundles for bedding over the wooden planks platform just 1' above the ground. But, we never saw the horse lying down. We enjoyed hitching the horse to the cart and tapping on the muzzle (very soft) with our hands.