Out here in North western Nevada, I see plenty of animals laying down for various reasons.
Horses lay down, roll in the dirt or grass and then nap. I watch the cattle more closely. Around here, their behavior is sometimes a weather gauge. If their laying down I prepare for lightning and thunder. Horses get twitchy when the air changes, and they detect more negative ions or something. The deer do the same thing. We have mustangs around here, and they lay down for comfort and leave a few standing to watch for predators. A lot of horses are smarter than their riders, if they're tired, they lay down if they can.