How do dolphins sleep?

Observations of bottlenose dolphins in aquariums and zoos, and of whales and dolphins in the wild, show two basic methods of sleeping: they either rest quietly in the water, vertically or horizontally, or sleep while swimming slowly next to another animal. read more

Adult male dolphins, which generally travel in pairs, often swim slowly side by side as they sleep. Females and young travel in larger pods. They may rest in the same general area, or companionable animals may pair for sleeping while swimming. read more

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. read more

Within a 24-hour period, each half of the brain gets about 4 hours of slow-wave sleep, according to a 2008 article in the journal Neuroscience & Biobehavioral Reviews, which also notes there's scant evidence among dolphins for rapid-eye-movement, or REM, sleep (the stage in which dreams typically occur in humans). read more