Horses can doze standing up and spend more time upright than other animals, but get their deepest rest, the so-called REM sleep, only when lying down.
In total relaxation, which rapid eye movement or dream sleep involves, the system of tendons and ligaments that keeps horses’ legs extended does not work. If a horse goes into REM sleep while it is standing, it tends to fall on its knees.
Wild horses lie down less, a fact that some researchers attribute to the need to be ready to flee predators. But others suggest they lie down less because during certain times of the year, notably winter, they eat day and night because less feed is available.
In summer, wild horses lie down a fair amount. When they do lie down, one usually remains standing as a sort of guard.