I have the suspicion that they're dreaming. If you'll notice, sometimes they'll "chew" in their sleep after "chasing" something down. It's adorable...watch the sequence sometime. I've even had kitties knead on my leg in their sleep...and some newborn kittens will suckle in their sleep. It's just kitty dreams! :o)
I love watching my cats when they are in deep sleep. Their legs shake, whiskers twitch..it's adorable. Not to mention, when they are in this mode I know they are at their most rested (since I rarely see them in deep sleep, I sometimes wonder when the heck they are getting good sleep at all).