Horses typically sleep standing up, though they do lie down during REM sleep. They stay upright even when unconscious due to ligaments and tendons within their legs that lock into place when their hips rotate slightly during rest. This system of ligaments and tendons is called the stay apparatus.
Horses' body weight is actually supported on three legs during sleep; one hind hoof relaxes and triggers the leg-locking mechanism, while the rest of the limbs hold the horse upright. During REM sleep, the period of sleep during which the brain is more active and twitching and other movements can occur, horses lie down or stretch out on the ground, probably to prevent falling. In herds, horses sleep in groups with a rotating watch-horse staying awake to alert the other members of danger. Adult horses sleep for three hours per day, while foals spend up to half the day asleep.