Nightmares and Cat Therapy

Sometimes cats wake up in the middle of a perfectly good nap, just to clean themselves, and then they drift right back to sleep. What's up with that? Are they having some disturbing nightmare about having a section of leg-fur out of place? That would be a tragedy. Good thing they wake up and take care of that -- otherwise they'd be haunted, I'm sure of it. My cat doesn't want to wind up in therapy.