Humans do very little when we sleep. This might not seem like a big deal, until you look to the dolphins as they sleep and swim with the pod. Or fur seals, who can sleep while watching out for sharks. Best of all there are birds, who can rest part of their brain to sleep while, yes, flying. While sleep for humans may seem straightforward — lie down, close eyes, count sheep — how it works is quite mysterious, even to the experts who study it. Why do we need to sleep at all?