Only one-half of a dolphin’s brain sleeps at a time. The other half that’s awake signals the dolphin to come up for air to prevent drowning.
Dolphins sleep by resting half of their brain at a time. This is called unihemispheric sleep. The brain waves of captive dolphins that are sleeping show that one side of the dolphin’s brain is “awake” while the other is in a deep sleep, called slow-wave sleep.