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Study: Elephant seals at sea sleep just two hours a day

A biologist at Scripps Institution of Oceanography has found that elephant seals take multiple 10-minute naps during their nearly 10-month foraging trips out to sea.

Like us, elephant seals need to sleep. And since they’re air-breathing mammals that are not buoyant, you may wonder how they manage to sleep when out to sea.

Jessica Kendall-Bar, a postdoctoral fellow at San Diego’s Scripps Institution of Oceanography, wondered the same thing.

“That breath-holding capacity allows them to forage deep below the ocean surface and do things like sleep below the ocean surface,” she said.


Kendall-Bar is the lead author of an article in the journal Science that describes the sleep patterns of elephant seals. She said elephant seals don’t sleep so much as they nap.

“These elephant seals are sleeping for very short periods of time,” she said. “And that only adds up to about two hours a day for these long foraging trips that last up to 295 days at sea.”

Jessica Kendall-Bar, a fellow at Scripps Institution of Oceanography, stands with Scripps Pier behind her. April 26, 2023
Thomas Fudge
Jessica Kendall-Bar, a fellow at Scripps Institution of Oceanography, stands with Scripps Pier behind her. April 26, 2023

Another finding is that the seals actually go into REM sleep during those cat naps. It’s the same full-body paralysis humans achieve during nighttime rest.

“At that time, they’re turning upside down and they’re actually falling in this beautiful sleep spiral pattern toward the ocean floor before they wake up at around 300 meters and swim back to the surface,” Kendall-Bar said.

Kendall-Bar began her research while pursuing her doctorate at UC Santa Cruz. She worked with an engineer that had created a device that could monitor brain activity in animals to find out if the elephant seals were actually sleeping.


She had to take this little neuro-logger, a piece of electronics, and design a housing for it,” said Dan Costa, a biology professor and one of Kendall-Bar’s advisors at UC Santa Cruz.

“So she designed a housing, then worked with some engineers to build a waterproof housing that she could put that device in, and then attach — she told me some 20-30 different wires — make all of that work and not leak.”

What Kendall-Bar learned is not just a scientific discovery. It allows humans to understand where and when seals sleep and protect them from the disruption of ships at sea.

“So I’m interested in working with collaborators who study other true seal species to help identify where these critical resting habitats are,” she said.

Humans have actually done a pretty good job protecting elephant seals. Costa said in the late 19th century, there were no more than 25 elephant seals. Now there are more than 200,000.