Night in the Ocean
At night, blue whales rest near the surface (~20 m deep), where they are neutrally buoyant and sing or sleep. As mammals, they need to come to the surface to take a breath of air approximately every 20 minutes, so they can't sleep for hours like landlubbers do. Instead, they put half of their brain to sleep at a time, and rest... until they are refreshed.
This type of sleep, called unihemispheric sleep, has also been documented in birds, allowing them to migrate long distances without stopping to sleep, or falling out of the air from exhaustion.
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