You don't see a horse lying down very often. Only during the middle of the day, only during specific weather and only when they are certain they can do it without being disturbed. Being herd animals usually on the lower end of the food chain, they are required to stand most of the time to survive. They sleep only two or three hours a day at most and that is usually done standing up in 15 minute increments. To achieve R.E.M. sleep however, they have to lie down. I was outside working the whole day and the sun was perfect so Buttons decided to grab a little snooze while she felt completely safe to do so.
She didn't need to worry, her faithful companion was always nearby to watch over her.