A pod of sperm whales is captured on film near the sunlit surface of the Atlantic ocean, off the Azores. Each animal is in calm, vertical repose and is indulging in a communal sleep-in, or drift dive.

Scientists believe that Physeter macrocephalus, which can measure up to 60ft in length and which has the biggest brain of any known animal, sleeps in a highly distinctive manner. According to researchers at St Andrews University, who tagged animals with recorders to follow their behaviour underwater, the sperm whale sleeps by taking short naps during slow, rhythmic dives. The animals are unconscious for no more than 10 to 15 minutes at a time over a few hours, however, and sleep, in total, for fewer than two hours a day. Thus sperm whales sleep less than any other wild mammal.

Unlike dolphins, which have been observed in captivity sleeping with only one side of the brain at a time, sperm whales switch off completely during these dives, the researchers added. "Many mammals show species-typical sleeping behaviour, such as dogs circling before lying down, lending support to the idea that sperm whales sleep during these drift dives," says Dr Patrick Miller, of the university's sea mammal research unit.

The sperm whale gets its name from the milky-white waxy substance, spermaceti, that is found in its head. This was originally mistaken for sperm. The animal is a record-breaker for many aspects of its behaviour. It is the largest living toothed animal and can dive to depths of three kilometres, making it the deepest-diving mammal. It is also the world's largest living predator. More to the point, the discovery that sperm whales can snooze while diving makes them the only mammals that can truly be said to drift off to sleep.

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