Most horses remain in the standing position because their weight places excess pressure on their internal organs when lying down. Adult horses only lie down for brief periods of time. Foals spend more time on the ground during naps until they get older.
Horses have two anatomical abilities that allow them to remain standing. One function is the "stay apparatus" of the forelegs. The horse engages the stay apparatus by shifting the hip and locking the patella into position. The "check apparatus" allows the hind legs to relax without collapsing.
When sleeping, an adult horse rests most of its weight on the two front legs and one hind leg. Horses enter a light sleeping phase in the standing position, which is an instinct left over from when they were wild animals. On average, horses spend four to 15 hours sleeping and several minutes to a few hours on the ground. Horses must also lie down to get their minimum REM sleep, which stands for rapid eye movement sleep. To achieve REM sleep, they must rest on the ground for one to two hours every three days.
Horses also lie down to sunbathe, and there are times when they rest in the sun together. One or two other horses stand guard as the others sunbathe. Horses lie down less frequently in snowy conditions, but some sleep in an outstretched position on the snow.