On land, when it spends its time mating and molting, the northern elephant seal sleeps for nearly 11 hours a day. But when it’s time to forage for food and it has to swim out to sea for months, the seal’s sleep pattern changes significantly.
But how does the seal sleep while in the water? Scientists also asked the same question, as these seals were always seen diving almost all the time. Could it be that they sleep while they dive?
To find the answer, scientists equipped 13 young female seals with a watertight EEG cap and sensors to monitor their brain waves, heart rates, and 3-D motion. The team, led by ecophysiologist Jessica Kendall-Bar, found that these seals do sleep while diving.
The seals first dive 60 to 100 meters below the surface. For a few minutes, they try to maintain themselves upright before REM and sleep paralysis kick in. Then they unconsciously flip upside down and gently descend in a spiral motion to depths where their natural predators do not normally go. After 5 to 10 minutes, the seals wake up from their power naps and swim back to the surface.
Scientists found that northern elephant seals sleep around two hours a day on average when they forage for food at sea.
It is worth noting that the sleeping habits of northern elephant seals are different from other marine animals like dolphins and fur seals who sleep only with half of their brains (one-sided sleep). Unlike other marine animals who try to be alert at all times, these seals leave themselves completely vulnerable.
The study of sleep behaviors across the animal kingdom is an essential step to understanding the function of sleep for all animals (including us humans).
(Image Credit: Jerry Kirkhart/ Wikimedia Commons)