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How many hours do you sleep 24 hours a day? Sleeping only four or five hours a day should already be the "limit" for most people, right?
Recently, scientists have just discovered that there is a mammal sleep pattern that is simply amazing: they only sleep an average of 2 hours a day during their long-distance travel to the deep sea, which lasts seven or eight months! This also allowed them to successfully challenge "the world's least sleepy mammal".
This animal is the giant beast of the sea, the elite diver - the elephant seal. In stark contrast, elephant seals sleep 10 hours or more a day when they stay on the beach.
The 2-month-old northern elephant seals (Mirounga angustirostris) are sleeping on the beach (Image source: Reference [2]; Credit：Jessica Kendall-Bar，NMFS 23188）
In a new paper published in the top academic journal Science, scientists use advanced remote monitoring technology to reveal in detail the sleeping miracle of elephant seals, many of which are known for the first time.
Sleep puzzles in the ocean
The question of when and how elephant seals sleep has puzzled scientists for a long time.
It's likely that you've seen how dolphins sleep in books like "100,000 Whys" as a child: the two hemispheres of their brains will sleep alternately, half asleep, half awake, and after a while, both sides will change shifts. In addition to dolphins, whales, sea lions, and fur seals also use this sleep strategy in the sea. However, elephant seals (and other seals) have not mastered the brain's single-hemisphere sleep method, and like humans, their entire brain goes to sleep when they sleep.
The sleep patterns of most mammals, including elephant seals and humans, are always in a state of sleep in both hemispheres of the brain at the same time (Image source: Reference [3]; Credit：Jessica Kendall-Bar）
Of course, it doesn't matter in the ground environment, but elephant seals need to stay in the open ocean for months during foraging and migration, so how to solve the problem of sleeping?
Nearly 30 years of tracking
To solve this mystery, marine mammal expert Professor Daniel Costa of the Institute of Marine Science at the University of California, Santa Cruz (UCSC) has been studying it for nearly three decades. He and his colleagues developed and used a variety of electronic tags to track the movement and diving behavior of wild northern elephant seals (Mirounga angustirostris) in the Pacific Ocean.
Professor Daniel Costa (right) with his subjects (Credit: The Costa Lab/UCSC)
In this study, Dr. Jessica Kendall-Bar, a member of Professor Costa's collaborative team, led the development of a specialized tool: using a special elephant seal hood to record the electroencephalogram (EEG) activity of these marine behemoths during diving. "We use the same EEG sensors that humans use when they are studying sleep in the sleep department, and use a removable flexible adhesive to secure the hood to make it waterproof." She explained.
In addition to EEG systems, small data loggers carried by elephant seals can simultaneously report time depth, acceleration, motion data, and more.
▲Sensor used to record elephant seal EEG (Image source: Reference [2])
Armed with the new equipment, the research team first recorded data on the brain activity of 13 juvenile female elephant seals while sleeping on the beach and during short-term dives in the bay. "I spent a lot of time watching elephant seals sleep," Dr. Kendall-Bar says enviably.
Based on the collected data, they then developed an algorithm that can accurately identify elephant seals from diving data whether they are awake or asleep, and distinguish what stage of sleep they are in, such as slow-wave sleep (SWS) or rapid eye movement (REM) sleep. Preliminary data suggest that elephant seals nap during dives for no more than 20 minutes at a time.
Immerse yourself in deep sleep
The team then began to actually track wild elephant seals for more than 7 months of free life at sea, and finally succeeded in deciphering how they sleep in the water. The researchers note that this is the first time that sleep patterns in free-roaming wild marine mammals have been recorded.
Records show that the elephant seals sleep an average of only 2 hours a day. This data allows them to equalize the long-standing record of African elephants for "sleeping least sleeping mammals."
Not only do they sleep less, but elephant seals also "fragmented sleep": every 30 minutes of deep dives and short naps of 10 minutes.
▲Data visualization reconstructs the trajectory of elephant seals diving while napping (Video source: Reference [1])
Despite this, elephant seals can go into a "deep" sleep for a short period of time. On the one hand, "deep" is reflected in the fact that its sleep state quickly enters slow-wave sleep (deep sleep), and then transitions to REM sleep. This complete REM sleep is something never seen in other marine mammals in the past. On the other hand, sleeping elephant seals continue to dive, reaching depths of nearly 400 meters. In shallow waters, even sink directly to the bottom of the sea and lie motionless.
And the most interesting thing is that when elephant seals are in the REM sleep stage, due to sleep paralysis - we humans normally do not dance because of dreams when we sleep, they cannot control their body posture, so their bodies flip and sink to the bottom of the sea in a spiral shape like swirling fallen leaves.
▲ When elephant seals enter REM sleep during deep diving, sleep paralysis will cause them to flip their bodies and sink in a "sleep spiral" (Image source: Reference [2]; Credti：Jessica Kendall-Bar）
Although the sinking looks comical, researchers familiar with elephant seals point out that this sleep strategy is very clever. Because fat, fat elephant seals are rare "fat" in the eyes of sharks, killer whales and other marine killers, once they surface in the open ocean, they are easily attacked by predators. Elephant seals only breathe for a minute or two between breath changes in two dives, and with the ability to hold their breath for an extended period of time, they can take naps and continue to sink during deep dives, just enough to avoid predators.
Researchers working to protect elephant seals also emphasize that the findings suggest that to truly protect elephant seals, in addition to caring about the habitat where they feed, it is also necessary to consider how to protect the habitat environment suitable for their sleep and rest.
Catching up on sleep is indispensable
Sleep is indispensable, and we must catch up on sleep after staying up all night, like seals.
Now, new research also explains why they sleep all day when they return to land: they have to pay off huge "sleep debts" they owe while at sea. However, such extreme changes in sleep duration are the first time scientists say they have been recorded in mammals.
If you usually have to sleep deprivedly like elephant seals in the sea for various reasons, you may wish to learn about elephant seals on land during the holidays and sleep for a few days!
Resources:
[1] Jessica M. Kendall-Bar et al., (2023) Brain activity of diving seals reveals short sleep cycles at depth. Science. DOI: 10.1126/science.adf0566
[3] Elephant seals drift off to sleep while diving far below the ocean surface. Retrieved Apr. 24, 2023 from https://www.eurekalert.org/news-releases/985876
[4] Elephant seals drift off to sleep while diving far below the ocean surface. Retrieved Apr. 24. 2023 from https://www.ox.ac.uk/news/2023-04-21-elephant-seals-drift-sleep-while-diving-far-below-ocean-surface