I've seen it twice. Scared the wits out of me both times. First time I was watching bighorn sheep on their winter range just outside the northern edge of Yellowstone Park. A bunch of rams fed over a little rise, but I could see one big ol' ram laying on the ground, his head resting to the side, propped up by his heavy horns. This was many years ago, and collecting dead horns was okay, so I thought I'd go over and pick up the head. I casually walked over to the dead ram and when I was about 5 yards away, the dead ram became airborne, swapped ends, and was standing facing me with a I'm-about-to-whip-your-butt, steam-blowing-blowing-out-the-nostrils look. To this day I wonder just how high my pulse rate went.
The other time was in Northern New Mexico about 10 years ago. I was hiking a little, isolated hill top, north of Tres Piedras and saw a dead 5x5 bull laying on the ground behind a juniper. Head tilted over, legs strecthed out, no sign of life. I told my buddy I was going to check out the dead bull. This time I actual was reaching out to grab one of the antlers, when the moutain of hair and antlers jumped off and trotted down the hill. Again, an involuntary cardio-stress test.
So, they do sleep.