Because horses are prey animals, they don’t like sleeping on the ground. Due to their straight backs, horses can't get up quickly. If a predator approaches a sleeping horse while it's lying down, the horse may not be able to get up fast enough to escape.
Thanks to specialized legs, however, a horse can lock its knees and fall asleep standing up — without falling over! A horse’s fear of predators also leads it to do most of its sleeping during the day.
Just because a horse can fall asleep standing up doesn’t mean it never lies down, though. Sometimes horses will take naps lying down during the day. This gives them a chance to rest their legs.
On a sunny day, you may see horses stretched out in a field soaking up sunshine. When you see a group of horses lying down, you may also find one horse standing up, guarding the others and keeping an eye out for intruders.
Many people are unaccustomed to seeing horses lying down. Some police stations report that on particularly sunny days — when horses are most inclined to stretch out for a nap — they will receive numerous calls from concerned people passing by who think the horses are dead. Of course, these horses are just taking a summer snooze!