Elephant Seals Take Extreme Power Naps in the Open Ocean

While foraging on deep dives, the marine mammals sleep for about two hours per day in short, ten-minute bursts

Sleeping Seals on a beach
Sleeping northern elephant seals on the beach at Año Nuevo State Park, California. Jessica Kendall-Bar

The African bush elephant holds the world record for the mammal that clocks the fewest hours of sleep per night, dozing off in bursts that together add up to just two hours. But now, scientists have discovered these animals may be rivaled by their marine namesakes: elephant seals. 

In a new study published last week in Science, researchers report that elephant seals have mastered an extreme power-napping strategy while in the open ocean—they’ll sleep for only about two hours per day in quick, ten-minute chunks.

Elephant seals spend about seven months of the year searching for prey at sea, diving up to 2,500 feet deep and spending up to half an hour underwater without air. The massive mammals come up to breathe for only a few minutes at a time between dives to avoid becoming the meal of a hungry shark or orca. In the ocean’s depths, the darkness helps them hide.

“There’s a bit of a conundrum,” Chris McKnight, an ecophysiologist at the Sea Mammal Research Unit at the University of St Andrews in Scotland who wasn’t involved in the study, tells NPR’s Ari Daniel. “If you’re diving all the time, if you’re spending 90 percent of your time at sea underwater without access to air, when the hell do you sleep?”

Unlike dolphins and sea lions, elephant seals cannot perform unihemispheric sleep—or sleeping with only one part of the brain while the other remains alert. So, scientists have long wondered when, if at all, elephant seals were getting their shut eye.

Sleep at sea: Brain activity of diving seals reveals short sleep cycles at depth

To learn more about the seals’ sleeping habits, Jessica Kendall-Bar, a researcher at the Scripps Institution of Oceanography, developed a novel swim cap-like device with electrodes to measure the animals’ brain activity and body sensors to collect movement and heart rate data. After running some tests on herself, she attached the head caps to 13 juvenile female seals—five in a lab and eight in the wild—and collected data over several days. This is the first study to record the brain activity of a free-ranging, wild marine mammal, per a statement.

She found that elephant seals could maintain their body posture as they were in their slow-wave sleep, the deepest phase of non-REM (rapid eye movement) sleep. But when the seals switched to REM, a phase of sleep when some muscles become temporarily paralyzed, they seemed to lose control of their bodies and began a distinctive spiral downward to the ocean floor, “like a falling leaf,” Kendall-Bar says in the statement.

“The thing I find remarkable is that any mammal would fall asleep while drifting hundreds of meters below the water surface,” co-author Terrie Williams, a comparative ecophysiologist at the University of California, Santa Cruz, tells BBC News’ Victoria Gill. “This is not light sleep but real paralytic, deep sleep that would have humans snoring. Remarkably, the seal’s brain reliably wakes them out of it before running out of oxygen… Imagine waking up on the bottom of a pool—it sends a shiver down the spine.”

Using the information they collected, the researchers created an algorithm to identify periods of sleep from the dive data. They then applied this algorithm to data previously gathered from more than 300 seals to get a population-level look at elephant seal sleeping habits. The team found that while the seals only slept about two total hours while out at sea, after returning to land, they snooze for more than ten hours at a time. 

“Northern elephant seals exhibit unparalleled flexibility in their sleep duration… No other mammal goes from sleeping about two hours a day for over 200 days to sleeping 10.8 hours a day,” Kendall-Bar tells the New York Times’ Annie Roth, adding that she hopes this research will aid conservation efforts. 

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

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