An article in Science on April 20th 2023 studying elephant seals has been recorded as the first instance of scientists using the brain waves of free wild marine mammals to determine their sleeping schedules.
The scientists involved recorded the time-depth profiles of 334 wild northern elephant seals, concluding that seals will take intermittent naps that span up to 20 minutes whilst submerged at depths of up to 300+ meters, even taking naps on the seabed. By nature, seals are always diving at 30-minute intervals and the minimal sleep they get is through what is known as ‘drift dives’.
Elephant seals remarkably sleep up to just 2 hours a day on average whilst they are on long foraging journeys at sea
Once in a state of deep sleep paralysis, elephant seals – who can sink to depths of up to 760 meters in some cases – will continue to spiral deeper down during their 30-minute dives. The seal’s brain will notably wake them out of their intermittent nap before they run out of oxygen to which they ascend to the surface and prepare for yet another dive.
As a result, elephant seals remarkably sleep up to just 2 hours a day on average whilst they are on long foraging journeys at sea that sometimes last up to 295 days in total. These unusual sleeping patterns rival that of the African Elephant, which holds the record for the least sleep (2 hours a day) among mammals. However, elephant seals are known to take advantage of the safety of land, sleeping up to 10 hours a day.
The process of sinking to depths of 377 meters to sleep is rooted in the survival instinct of seals to avoid predators who inhabit the ocean’s surface, particularly killer whales and sharks who prey on seals specifically. By inducing sleep paralysis and descending deep into the ocean, seals are able to safely induce quick naps on their foraging expeditions.