Why do I tire of counting sheep, when I'm far too tired to fall asleep?

With a blue background in my mind, precisely the same color as my flannel sheep-covered sheets, I conjur up a length of fence. It is a white fence, several planks in length, and rather short. Can't go too much longer or the sheep wouldn't be able to make it over. There might be some grass, there might be just the fence hanging in the blue; I never really bother to look at the spot where the ground would be. It's really only the sheep that matter, right? One by one, the sheep fade in at their last running step, then float in a gentle arc over the white fence, then fade away during their first running step at precisely the same instant the next one fades in. I don't watch the fading too closely; there is always one fading in and one fading out, but I focus on the jumping motion, I count that part. After a while I tire of counting these sheep. The new batch of sheep don't jump or move by themselves at all, they are not alive like the others. These ones are attached to one of those hanging, toy things that sit above babies' cribs. They move in a perfect circle around the fence, but I only see them once the reach the ground-level and while the soar over the fence. Then they go under ground again while the next sheep gets rotated in. After another three hundred of these, I tire again. The new sheep are more acrobatic than the last; dropped in from the side of my mental screen, they bounce off the carefully positioned trampoline over the fence, hit the trampoline waiting for them on the other side, and bounce out of the screen again as the next one comes in, like a sheep-counting screen saver. Why hasn't anyone made one of those?
My sheep, all varieties, always travel from right to left over the fence, quite the opposite of reading. Anybody else?