Seals snooze during 20-minute ‘sleeping dives’ to avoid predators

Sleeping elephant seals on a California beach

On the shore, elephant seals sleep for about 10 hours a day. But while at sea, they sleep just two hours daily. Jessica Kendall-Bar

Northern elephant seals are challenging the world record for the mammal that sleeps the fewest hours a day. The current record holder is the African elephant, who rests a measly two hours daily. Now scientists report that elephant seals also sleep an average of two hours a day when they’re out at sea and do this by splitting their slumber into a series of nap-like “sleeping dives.” 

These findings were published today in the journal Science

Elephant seals divide their time between land and sea, though it’s unequal. They spend an average of seven months out of the year in the open ocean and only resurface to breed, molt, and rest. Because they spend so much time in open waters, scientists figured these marine mammals must have developed some way of getting the sleep they need while avoiding detection from predators like the orca whales and great white sharks. But exactly how they do this has been poorly understood.

[Related: Take the best naps, with science]

One challenge in understanding the sleep behavior of elephant seals is finding a device that’s both waterproof and can handle deep-sea pressure. To overcome this, the study team created a flexible head cap that can respond to seals’ twisting and flexing motions. It’s also made up of a synthetic rubber called neoprene, the same material found in wetsuits. The scientists used this cap to monitor the seals’ brain activity, heart rate, and three-dimensional spatial movement.

Scientists outfitted 13 wild seals with the cap. Five were kept in a lab, while the other eight could freely roam around Monterey Bay, California. The EEG recordings collected from the head cap represented brain activity during different sleep stages. 

“We can take the data and use it to recreate what the sleeping dives look like, and also what’s happening within the animal brain, how fast its heart is beating, etcetera,” says lead study author Jessica Kendall-Bar, a Scripps postdoc scholar at the University of California, San Diego.

How do seals sleep in the ocean?

Elephant seals sleeping in the ocean shallows
Sleeping 2-month-old northern elephant seals in the shallow lagoon at Año Nuevo State Park, California. Jessica Kendall-Bar

The collected data indicates elephant seals sleep about two hours a day while at sea, though not all at once. When it was time to get a little shut-eye, seals dove hundreds of meters below the surface—the maximum depth was about 1,200 feet—where they would take quick naps lasting less than 20 minutes. 

Kendall-Bar says this “degree of flexibility and sleep duration has really only been demonstrated in birds and is pretty much unprecedented in mammals.”

Dive naps likely evolved as a way for seals to avoid getting attacked since their natural predators lurk near the surface, explains Kendall-Bar. They are also more vulnerable than other marine mammals when resting because they undergo bilateral sleep. This means both halves of the elephant seal’s brain rest when they sleep. Human beings also experience bilateral sleep. 

Meanwhile, fur seals and sea lions experience unihemispheric sleep—one brain hemisphere rests while the other stays awake and monitors for predators.

Different stages of underwater sleep

The study data suggests seals go through one complete sleep cycle during each nap-like “sleeping dive.” When these brief sleep cycles end, the seals return to the surface. This process allows them to rest at depths with lower predation risk while staying vigilant in more dangerous waters. 

Elephant seal sleep spiral in the ocean in a blue and purple diagram
When seals go into REM sleep during dives, sleep paralysis causes them to drift downwards in a “sleep spiral,” as seen in this diagram. Jessica Kendall-Bar

During nap dives, the seals entered slow-wave sleep while maintaining an upright posture. They then turned upside down while their sleep cycle transitioned from slow-wave sleep to rapid eye movement (REM) sleep. 

“The sleep state of the animal is actually reflected in its movement through the water,” explains Kendall-Bar. 

Once the cycle was complete, the seals immediately woke up and returned to the surface to find food.

[Related: Pendulums under ocean waves could prevent beach erosion]

Since muscle paralysis from REM sleep leaves seals exposed and defenseless, they took the shortest naps possible and compensated for the lack of sleep after reaching land again. As a result, the seals slept five times longer ashore than they did in the water. Some seals even slept up to 14 hours a day on land.

“What really stood out for me is the fine-scale analysis the researchers did to identify the different sleep states and how they were able to translate this analysis to estimate sleep patterns in seals at sea,” says Cassondra Williams, a comparative physiologist at the National Marine Mammal Foundation who was not involved in the study. “This will be an important tool for future behavior studies of pinnipeds freely diving at sea.”

Most diving naps took place just near the shore. While northern elephant seals are not currently endangered (in the 1800s, they were almost hunted to extinction), Kendall-Bar and her team are concerned that shipping traffic and traps on the seafloor may be disturbing their habitats. Understanding when and where seals slumber could help conservation efforts and ensure seals get all two hours of their beauty sleep.