"Dolphins are known to be one of the smartest animals on the planet—possibly because they can conserve their brain power. Since they must always be on the lookout for predators, the marine mammals have developed a neat trick of maintaining partial consciousness even as part of their brain sleeps.
Researchers have tested whether this "half-sleep" negatively impacts the animals' alertness during the day, but have found that even after five days of having their nocturnal alertness constantly tested, they've remained as alert and perceptive as ever."
From "Interesting Facts".
