Elephant Seals in the North Ocean Sleep for Only Two Hours a Day

According to researchers, northern elephant seals are experts in taking power naps. While on long trips into the sea, the seals have been observed sleeping for less than 20 minutes at a time. The seals typically rest for just two hours in total per day, even though they are swimming offshore for months. This measurement rivals that of African elephants as the least amount of sleep measured for mammals. Ecophysiologist at the University of California, San Diego, Jessica Kendall-Bar, said the mapping of these sleep behaviour extremes is crucial in understanding the evolution and function of sleep across the animal kingdom, including humans. It could also help conservation efforts aimed at protecting the areas frequented by these seals.

These elephant seals spend most of the year in the Pacific Ocean foraging around the clock for fish, squid and other food as they sustain their massive bodies that can weigh as much as a car. To avoid sharks and killer whales, which are more common at the surface, the seals come up for air for a couple of minutes between 10- to 30-minute deep dives. Niels Rattenborg, a neurobiologist at the Max Planck Institute for Biological Intelligence in Seewiesen, Germany, who was not involved in the study, said it was not previously known if and how the seals sleep.

Kendall-Bar's team developed a water-resistant EEG cap for the seals and other sensors to track the brain waves, heart rates and 3-D movement of 13 young female seals. They captured their naptime brain waves by monitoring their EEG readings while sleeping. The researchers also took two seals from coastal Año Nuevo State Park north of Santa Cruz, Calif. strapped with sensors and released them at another beach approximately 60 kilometres away. As the seals swam home, they had to cross the deep predator-filled waters of the Monterey Canyon, much like they do during long foraging trips. The team was able to match the seals' EEG readings to their diving motions to show just how northern elephant seals sleep on long voyages.

The seals first dive 60 to 100 meters under the surface and then relax into a glide as they fall asleep. As they enter slow-wave sleep, the seals keep holding themselves upright for several minutes. However, once they enter REM sleep, sleep paralysis takes over. The seals flip upside down and spiral gently towards the seafloor. Seals can go hundreds of meters down during these naps, going deeper than their predators prowl. When the seals wake after a five to ten-minute nap, they swim to the surface. The whole routine takes approximately 20 minutes.

The researchers could identify sleep patterns in the dive records of 334 adult seals outfitted with tracking tags from 2004 to 2019 by looking for the distinct sleep dive motion. These sleep patterns showed that, on average, the seals were sleeping for about two hours per day while on months-long foraging missions. When the seals return to land, they sleep for nearly 11 hours per day while they mate and molt. They can indulge in long, beachside siestas without worrying about predators.

It is interesting to note that the sleep habits of northern elephant seals are different from how other marine mammals sleep in labs. Kendall-Bar said, "Many of them … sleep in just half of their brain at a time." This half-awake state allows dolphins, fur seals and sea lions to practice constant vigilance, sleeping with one eye open. Kendall-Bar further commented that it is fascinating that elephant seals are capable of this type of sleep without one-sided sleep. The seals essentially turn both halves of their brains off, leaving them vulnerable. However, it seems the key to enjoying such deep sleep is sleeping deep in the sea.

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