How do dolphins sleep?

Biologists observing both wild and captive dolphins have noticed that they swim around in circles when they are asleep. They can do this because only one-half of their brain actually sleeps at any one time. Researches suspect that this behavior serves to keep individual member's of a group together. When dolphins are awake, they communicate by making whistling sounds which enable them to find one another easily. But when they are asleep, such sounds could easily draw unwanted attention from predators. However, if all the members of a group swim steadily and silently around in circles, they can still stay together but without attracting uninvited visitors.