While we're nitpicking, I've never known anyone (human or otherwise) to be able to hold their eyebrows in a perfectly fixed position while sleeping. The whole point of sleeping is to relax and recharge your body, so the fact that the muscles are acting to allow the eyebrows to be held like that seems a bit strange to me. Unless it's some sort of cartoon gimmick. As Helm already stated though, there's already tons of things going in the animation that just slaps the viewer in the face with the message that THE CAT IS SLEEPING (the "zzz"s, the slow breathing/body movement, the position, the pillow, the bubble, the closed eyes, and so on and so forth).