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Elephant Seals: A Short Sleep in the Deep

If you’ve ever seen elephant seals lying on a beach, you might think all they do is sleep. But now scientists have discovered that when elephant seals are in the ocean, they hardly sleep at all. On average, the seals sleep just two hours a day.

Elephant Seals
     Northern elephant seals are huge marine mammals that live in the Pacific Ocean. They can weigh up to 4,000 pounds (1,800 kilograms). They get their name because large male elephant seals have a snout that looks like an elephant’s trunk.

Elephant seals spend most of their time in the ocean. For about seven months of the year, they are in the water constantly.

Elephant seals sleep about 10 hours a day on the beach, but during months-long foraging trips at sea they average just 2 hours of sleep per day. These 2-month-old northern elephant seals are sleeping on the beach at Año Nuevo State Park. (Photo by Jessica Kendall-Bar, NMFS 23188)
Elephant seals sleep about 10 hours a day on the beach. But when they’re at sea for months, they average just 2 hours of sleep a day. Above, 2-month-old northern elephant seals, sleeping on the beach at Año Nuevo State Park in California.
(Source: Jessica Kendall-Bar, NMFS 23188, UC Santa Cruz.)

Even though elephant seals are large animals, they have to be careful in the ocean. Near the surface, predators like sharks and orcas are a constant danger.

So elephant seals spend most of their time diving deep underwater, looking for food. They can dive as deep as 2,500 feet (750 meters) below the surface, and can hold their breath for over an hour. Usually, they will dive for about 30 minutes, then come up for air for just a couple of minutes before diving again.

Scientists have wondered how elephant seals can sleep while they’re at sea, since they are almost constantly diving.

Animals like dolphins and sea lions have a special trick. One half of their brain can sleep, while the other stays awake. But elephant seals are like humans – both halves of the brain need to go to sleep at the same time.

Cetaceans (whales and dolphins) and otariids (fur seals and sea lions) keep one side of their brains awake while the other is asleep (unihemispheric sleep). In most other mammals, including phocids (true seals) and humans, both hemispheres of the brain are asleep at the same time. (Graphic by Jessica Kendall-Bar)
Scientists wondered how elephant seals can sleep while they’re at sea. Dolphins and sea lions have a special trick – half their brain can sleep, while the other stays awake. But elephant seals are like humans – both halves of the brain need to go to sleep at the same time.
(Source: Jessica Kendall-Bar, UC Santa Cruz.)

So how do they sleep?

To find out, a scientist named Jessica Kendall-Bar created a special cap that could record the elephant seals’ brain activity. The cap also collected information on the elephant seals’ heart rates, how their bodies were moving, and how deep they were. Dr. Kendall-Bar and her team put the caps on thirteen young female elephant seals.

Humans go through stages when they sleep. Some of the time, when you’re sleeping, you can control your body. But when you’re sleeping deeply and dreaming, your body is temporarily paralyzed. That keeps you from hurting yourself when you’re active in your dreams.

The scientists learned that elephant seals sleep in short bursts while they are diving. They only sleep for about 10 minutes at a time.

The elephant seals had sleep stages similar to humans. As they began to sleep, the seals could control their bodies, gliding down smoothly. But when they were deeply asleep, they simply began to fall, spinning in a circle “like a falling leaf”.

When elephant seals go into rapid-eye-movement (REM) sleep during deep dives, sleep paralysis causes them to turn upside down and drift downwards in a “sleep spiral.” This data-driven graphic shows sleeping postures every 20 seconds, with accompanying 30-second segments of EEG traces in the background. (Graphic by Jessica Kendall-Bar)
Elephant seals sleep in short bursts while they’re diving – about 10 minutes at a time. As they began to sleep, the seals could control their bodies. But when they were deeply asleep, they simply began to fall, spinning in a circle “like a falling leaf”.
(Source: Jessica Kendall-Bar, UC Santa Cruz.)

The seals woke up in time to go up for more air. In shallower areas, the elephant seals sometimes woke up on the sea floor.

Using the information they gathered from the cap-wearing seals, the scientists created a computer program to study older records of elephant seal trips. This allowed them to get information on over 500,000 “sleeping dives”.

The scientists found that elephant seals only sleep for about two hours a day when they’re in the ocean. That means that the rest they get once they return to land is very important. Back on land, they sleep for about 10.8 hours a day.

The researchers hope that their work will help to protect the places where elephant seals sleep when they’re on land.

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