The Fascinating Sleep Habits of Northern Elephant Seals in Deep Waters.

Northern elephant seals sleep while drifting hundreds of meters below the sea surface, at depths where their predators do not usually lurk. Researchers tracked the animals, recording their brain activity as the seals swam for thousands of kilometers. The mammals, which reach depths of up to 2,500ft, sleep for only two hours per day in what the researchers describe as "nap-like sleeping dives". The study has drawn "nap maps" for seals, suggesting that sleeping areas may be as important as active hunting areas for these animals.