When it comes to ocean diving, no seal can compete with the elephant seal. While on the hunt for their deep-dwelling squid and fish prey, these blubbery behemoths hold their breath for up to two hours and can plunge a mile beneath the surface; they skirt hungry orcas and withstand pressures that would flatten a junkyard car.
These epic dives help the seals survive long stints at sea, including the thousands-mile-long foraging trips they embark on each year, some of which take the animals from the west coast of North America to the middle of the Pacific and back. The animals swim and dive almost constantly for about seven months straight, never once going ashore to rest. And for decades, researchers such as Daniel Costa, a marine biologist at UC Santa Cruz, couldn’t help but marvel at the marine mammals’ stamina—and wonder how, and when, they possibly slept. There were only two possibilities, Costa told me: “They must sleep somewhere out there. Or maybe they don’t.”
The truth, it turns out, seems to be a little bit of both. Costa and his colleagues, led by his former graduate student Jessica Kendall-Bar, have found that, during the foraging trips, elephant seals average just two hours of sleep a day, broken into five- to 10-minute naps as the animals dive. The new findings represent the lowest daily-sleep total ever definitively recorded in a mammal—and are the latest in a long series of animal studies that show there’s simply no single way to sleep.
The human approach to sleep is, frankly, boring: eight-ish hours each day in quiet repose, usually in one continuous go. But most other creatures can’t afford that luxury when there’s so much else to do—and so many dangers afoot. Cows chew while drowsing; frigate birds fly while they snooze. Dolphins put just half their brain to bed at a time so they can remain partially alert. Some evidence even suggests, albeit indirectly, that elephants, like elephant seals, may clock just two hours of daily shut-eye for months on end.
Scientists can’t definitively explain why animals take such varied approaches to sleep; even the evolutionary purpose of the behavior remains a fiery academic debate. And the basic data have been very hard to come by, in part because directly measuring electrical activity in the brain—the gold standard for sleep research—is incredibly difficult in the wild, where animals have diverse lifestyles and very differently shaped heads. Watery environments pose a particular puzzle, and for years, the dozing habits of marine mammals were “basically inaccessible to science,” says Eric Angel Ramos, a marine biologist at the University of Vermont.
The elephant-seal study marks the first time researchers have broken through that barrier in the wild, according to Kendall-Bar (now at UC San Diego’s Scripps Institution of Oceanography), thanks to a new monitoring system that she designed. After quite a bit of tinkering, and even some experimentation on her own skin—“I went to the beach and stuck some electrodes on myself,” she told me—she was able to build a fleet of submersible sensors to monitor the animals’ heart rate, muscle and eye movements, and brain activity, including a swim-cap-like hat that could fit painlessly onto an elephant seal’s head.
The seals, Kendall-Bar discovered, were sandwiching snoozes into their dives. After spending a couple of minutes actively plunging into the ocean’s lower layers, the animals would drift off, continuing to glide downward as they dozed. A few minutes later, they’d enter REM sleep, a phase in which animals tend to lose muscle tone. Still descending, the seals, now belly-up, would begin to pirouette in what Kendall-Bar calls a “sleep spiral” as the churn of the ocean rocked them back and forth. Many of the animals would eventually end up napping motionless on the ocean floor until they woke, at which point they’d surge back up to the surface to breathe.
That these animals allow themselves multiple minutes of underwater REM sleep—a hyper-vulnerable state of paralysis—is a bit of a surprise, especially given how many sea creatures harbor a taste for fatty seal flesh, experts told me. But that just “emphasizes how important sleep is,” says Kathleen Reinhardt, a sleep researcher at the University of Calgary. REM, for instance, may prep the brain for the return to wakefulness, cutting down on the grogginess that could, say, otherwise prevent a seal from dodging a ravenous shark or swimming toward the surface to breathe, says Adhil Bhagwandin, a sleep researcher at the University of Cape Town.
One of the most impressive things about the seals’ underwater M.O. is that they manage all of their deep-sea naps without breathing, allowing their blood-oxygen concentration to fall to a level that would put humans in dire straits, Costa told me. Even when ashore, the animals sometimes hold their breath while they doze, Kendall-Bar said, as if practicing for their eventual return to sea. Cluing in to how the seals cope with that respiratory predicament might someday help researchers better understand or even better treat a condition called sleep apnea, which causes people to briefly stop breathing overnight.
Before researchers can draw such connections, though, they’ll need to figure out more of the mechanics of seal sleep. When on land, the animals snooze an average of 10 or 11 hours a day. And seals that do their diving exclusively in landlocked research settings will doze for six hours a day—perhaps because, in a food-rich, predator-free environment, they can. It’s a reminder that sleep patterns can be driven by both inborn biology and environment in animals and humans alike. “We have a sleep epidemic going on in industrial society,” with disorders aplenty, Reinhardt told me. Meanwhile, certain hunter-gatherer societies have been found to routinely snooze less than urbanites do—roughly six hours a night—with no apparent ill effects on their health.
Take that, plus what elephant seals accomplish at sea, and our modern sleep habits might start to look suspect. But duration can be deceiving, Bhagwandin told me. Our long periods of sleep may be less an indication that we need excessive rest and more a clue that restoration can be packaged into many forms. Perhaps elephant seals aren’t snubbing sleep and are instead just tackling the task very efficiently. And as much as these animals “shake up notions of how sleep functions,” Ramos told me, they can’t truly break the rules of sleep—not when, time and time again, the world’s wild creatures show us that there are no rules to break.