biologist observing both wild and captive dolphins have notices that they swim around in circles when they asleep. They can do this because only half of their brain sleeps at any one time. Researchers suspect that this behaviour serves to keep individual members 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 sound could easily draw unwanted attention from predator. However, if all the member of the group swim steadily and silently around in circle, they can still stay together but without attracting uninvited visitors.
there no picture of dolphin that sleep, so, i'm just upload this kind of picture.
p/s: just for general knowledge.