Most dolphin species sleep by resting one hemisphere of their brain in what’s called slow-wave sleep. During this state, they stay alert enough to keep breathing and look out for predators. The Indus river dolphin sleeps differently. Since the animal lives in waters with powerful currents and a great deal of floating debris, the dolphin must stay alert to avoid becoming injured. To stay safe, the dolphin sleeps in short four to 60 second spurts.