How do dolphins sleep without drowning? Is it true that they sleep with one eye open?

Dolphins, being air-breathing mammals, have to resurface every now and then to breathe fresh air. So, one wonders when and how they manage to get sleep.
Dolphins are a small group of whales and like other whales they have lungs instead of gills. They breathe through a single nostril called the blowhole located on top of the head. The blowhole is opened during their frequent trips to the surface to expel and inhale air.

Even though respiration has to continue with not more than few minutes’ respite between breaths, dolphins sleep an average of eight hours a day, but only allow one half of their brain to doze off at a time. They can not ‘switch off’ both hemispheres of their brain and have deep sleep underwater, or they would drown. So sophisticated is the dolphin’s sleep system that it naps at the surface of the water, with half of its brain alert and the opposite eye open, while the other half of the brain is asleep with the opposite eye closed. (See photo above.) During sleep, the dolphin alternates position to allow each half of the brain to get its turn. In effect, the dolphin is awake and asleep at the simultaneously.

Additional reading:
Dolphin (Wikipedia)

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