Why do dolphins sleep with half their brain awake?

When dolphins sleep, only one hemisphere of their brain rests, while the other half remains on alert. This ability to "turn" the cerebral hemispheres to partially sleep is known as unihemispheric sleep and, as scientists from the National Marine Mammal Foundation of the United States have just demonstrated in a study published in the journal PLOS One , it allows them to remain fully active and retain their full powers for at least 15 consecutive days.

The scientists worked with a male and a female who were able to perform echolocation tasks – emission of sound waves to locate themselves and determine the position of objects – for 5 days without any signs of fatigue. The female dolphin prolonged this state for another 10 days. The researchers did not continue in the experiment, so it is unknown how long the cetacean could have endured without showing signs of fatigue .

Until now, scientists thought that the ability to sleep with one hemisphere of the brain had evolved to make it easier for dolphins to breathe on the surface of the water. The new study indicates that the need to remain vigilant for a long time could also play a key role, "perhaps indefinitely", according to Brian Branstetter, who defines dolphins as "permanent sentinels of the sea."