Oooh, the sleep thing is what gets at me. Sleep isn't necessarily for the human brain; the human brain can run off just being rested in a low activity state like say closing your eyes, but not falling asleep. Sleep is to rest the body; when a person sleeps the muscles have a chance to unwind and relax without having to be strained. This would mean that the "zombies" have limits. The dead don't need to limit themselves. They also have an active cardio-vascular system that keeps blood flowing to their nervous system and throughout the body. Whenever you see them in a scene without them noticing any survivor's they groan and gurgle in a breathing kind of manner which is not a form of communication *reference to the scene when a group of zombies surrounds the hummer while Saul and Angel are in the store and one of the zombies comes in*. They also seem to have a basic understanding of technology like the sensor doors on the store (not that they know how to use it, just that some doors already open for them conveniently). I get this wonderful picture in my mind of a zombie being pleasantly surprised and smiling and giggling at the open door like a toddler.