For humans, sleep is simple: lie down, close your eyes and switch off for the night. For many animals, switching off completely would be dangerous. A sleeping animal cannot watch for predators, search for food or, in some cases, even breathe. Over millions of years, different species have found surprising solutions, and their habits show that sleep is far more flexible than we once thought.
Dolphins offer the best-known example. Because they must rise to the surface to breathe, they cannot fall into the deep sleep that humans enjoy. Instead, they rest one half of the brain at a time, a pattern known as unihemispheric sleep. While one half sleeps, the other stays alert, and the eye connected to the sleeping half closes. Research on captive dolphins and killer whales suggests that mothers and their newborn calves sleep very little during the first weeks after birth, perhaps because the calves need constant protection.