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A new analysis of elephant seal brainwave patterns has revealed that these mammals take short naps while holding their breath on deep dives, averaging just 2 hours of sleep per day during their long trips at sea. The findings have been published today in the journal Science.

Sleeping 2-month-old northern elephant seals on the beach at Año Nuevo State Park, California. © Photo by Rachel Holser. NMFS 23188.

For many years, researchers have been mystified about how the northern elephant seal, (Mirounga angustirostris), one of the world’s elite divers, gets enough sleep when it forages at sea. During these trips, which can last up to 8 months, the seals are constantly diving, both to avoid predators such as killer whales and because their colossal energy needs compel them to keep searching for food. But unlike cetaceans and ‘eared’ seal species (such as sea lions), elephant seals cannot perform unihemispheric sleep, where one half of the brain rests and the other half remains alert.

This left an intriguing, unanswered question – when and how do elephant seals sleep?

The answer has now been revealed by a new study where, for the first time, scientists recorded the brain activity of a free-ranging, wild marine mammal. The results show that elephant seals take short naps while diving deep below the ocean’s surface. Furthermore, wild seals average just 2 hours of sleep per day when at sea – rivalling the record for the least sleep among all mammals, currently held by African elephants.

Read the full story on the University of Oxford website.