Elephant Seals Take Power Naps During Deep Ocean Dives

It long seemed as though African elephants were the champions of the all-nighter. They can get by on about two hours of sleep. Other mammals need much more, like koalas (20 hours) or you (at least seven plus at least one strong cup of coffee).

But the largest living mammals on land have some competition at sea. Northern elephant seals are also able to sustain themselves on about two hours’ sleep, according to a study published Thursday in the journal Science. The study found that Northern elephant seals sleep far less at sea than they do on land, and the z’s they do catch at sea are caught hundreds of feet below the ocean’s surface. The study’s authors believe that sleeping in the deep allows the seals to power-nap without being eaten by prowling predators.

Northern elephant seals, which are found along the West Coast, are champion divers that can descend to depths of 2,500 feet and stay under for about two hours. They are not as big as elephants, but males can weigh as much as a car and stretch 13 feet long. To maintain their blubbery bulk, Northern elephant seals must spend around seven months at sea each year, gorging on fish and squid.

During these epic voyages, the seals are vulnerable to predation by great white sharks and killer whales. Some marine mammals, such as dolphins and fur seals, can rest half of their brain at a time. This type of slumber, known as unihemispheric sleep, enables some mammals at sea to snooze with one eye open, literally, which prevents predators from catching them off guard. However, elephant seals sleep like us, shutting down their brains completely.

Jessica Kendall-Bar, now a postdoctoral fellow at the Scripps Institution of Oceanography in San Diego, wondered how Northern ​​elephant seals managed to sleep, given how much time they need to spend eating and avoiding being eaten while at sea.

To find out how elephant seals avoid waking up in the maw of an orca or a shark, Dr. Kendall-Bar worked with colleagues at the University of California, Santa Cruz, to design a device that could monitor the seals’ brain waves, heart rates, dive depths and movement. The device is noninvasive and fits atop the seal’s head like a swim cap. The team attached the devices to the heads of several seals and monitored their sleeping habits for five days. The data collected by the devices revealed a bedtime routine unlike any other.

“They dive down, stop swimming and begin to glide,” Dr. Kendall-Bar said.

As they go deeper, their brain activity starts to slow.

“Then they transition to REM sleep, where they flip upside down and spin in a circle, falling like a leaf,” she said.

While in rapid eye movement, or REM, sleep, which is the deepest stage of sleep, the seals stayed upside down, oblivious to their slow descent.

After sleeping for around 10 minutes, the seals would suddenly wake up and make their way back to the surface. During these sleep dives, some seals sank over 1,000 feet, sometimes finding themselves on the seafloor.

The seals Dr. Kendall-Bar and her colleagues monitored took several sleep dives each day, providing them with around two hours of sleep in total. When Northern elephant seals haul out on land to breed and molt, they sleep for over 10 hours a day. During that time, the seals aren’t eating, which may explain their need for extra sleep.

“Sleep is an adaptive trait,” said Jerome Siegel, a professor of psychiatry at the University of California, Los Angeles, who studies the evolution and function of sleep. “Animals have evolved to sleep in certain situations and not in others.” It makes perfect sense, Dr. Siegel said, that elephant seals would limit the amount of time they spend sleeping while at sea to make the most of their food intake and reduce the amount of time they are vulnerable to predators.

That doesn’t make seal sleep habits any less impressive.

“Northern elephant seals exhibit unparalleled flexibility in their sleep duration,” Dr. Kendall-Bar said. “No other mammal goes from sleeping about two hours a day for over 200 days to sleeping 10.8 hours a day.”

Dr. Kendall-Bar hopes her team’s findings will aid in the protection of marine mammals.

“Learning more about where, when and how marine mammals sleep at sea can help scientists improve the management of their critical resting habitats,” she said.