Elephant seals drift downwards into the ocean in a "sleep spiral" to catch up on sleep while on months-long foraging trips but are programmed not to drown, according to a new study.
The seals fall into sleep during deep dives of up to 377 meters, which is around 1,235 feet, to avoid predators.
They spiral downwards for about 10 minutes at a time during half-hour dives, and they sometimes even catch some sleep on the sea floor, according to new findings published in Science.
The study marks the first time scientists have studied the brain waves and recorded the sleeping habits of a free-ranging, wild marine mammal, according to the University of California, Santa Cruz.
The study examined the crucial nature of sleep for mammals, and pointed out that marine mammals "encounter especially challenging conditions for sleep when they are at sea."
"For years, one of the central questions about elephant seals has been when do they sleep," Daniel Costa, director of the UCSC Institute of Marine Sciences, said.
The lab used tags to track the movements of elephant seals in the Año Nuevo Reserve when the animals head out to the Pacific Ocean for months at a time.
"The dive records show that they are constantly diving, so we thought they must be sleeping during what we call drift dives, when they stop swimming and slowly sink, but we really didn't know," Costa continued.
Professor Terrie Williams, from UC Santa Cruz, told BBC News that it was "remarkable" that any mammal would fall asleep while drifting hundreds of feet below the water's surface.
"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," said Williams.
African elephants currently hold the title of mammal that sleeps the least, at just two hours a day, but these new findings show that elephant seals "rival the record," according to UCSC.
Killer whales and sharks attack elephant seals when they are at the surface of the ocean, which is why they spend so little time near the top and only take a short time breathing at the surface between dives, per UCSC.
"They're able to hold their breath for a long time, so they can go into a deep slumber on these dives deep below the surface where it's safe," said Jessica Kendall-Bar, who led the study.
The scientists fitted neoprene headcaps with electroencephalogram (EEG) sensors to record the 13 young female seals' brain activity.
"We used the same sensors you'd use for a human sleep study at a sleep clinic and a removable, flexible adhesive to attach the headcap so that water couldn't get in and disrupt the signals," Kendall-Bar, a postdoctoral fellow at UC San Diego's Scripps Institution of Oceanography, said.
The recordings showed the diving seals going into a sleep stage known as "slow-wave sleep" before transitioning into REM sleep, which leads to a kind of "sleep spiral" or sleep paralysis, experts found.
Elephant seals do get a lot of sleep when they are on land – about 10 hours – scientists said, which makes their sleeping pattern "unusual."
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