Sleep Odities

"And this isn't even going into the subject of species that sleep with only half of their brain at a time, in order to keep one eye and half the brain open to look out for predators. Mallards, for example, that are stuck on the edge of their group at night keep their outward facing eye, and the half of the brain that responds to it, preferentially awake. As more oddities, dolphins can swim while sleeping and some birds can fly."