Marine mammals cannot close their eyes and sink to the bottom during the night. They need to come to the surface from time to time to get air. Dolphins and whales are mammals, so they cannot breathe underwater. They absorb air from the surface and then sink into the water for a certain period of time. To take in oxygen, dolphins use an organ called spiracles (similar to a human nose), and whales use an opening in their heads to draw air into the trachea and then fill the lungs with it. They also cannot swim on the surface of the water and cannot hibernate, as this would expose them to danger from predators and cause heat loss.

How do you sleep when you’re a dolphin?

One solution is to turn off one half of the brain at a time. This is called unihemispheric sleep, which allows marine mammals such as dolphins to rest offshore. Unihemispheric sleep is invaluable to these animals because it allows them to maintain a low activity level by sleeping half of their brain at once and using less oxygen.

Dolphins are the most studied marine mammals capable of this sleep style. Brain scans of captive dolphins show that one hemisphere is in a slow, deep sleep while the other hemisphere is awake, allowing the animals to literally sleep with one eye open. This sleeping style is common among cetaceans, a group of mammals that includes dolphins, whales, and porpoises.

However, this is not unique to them. We know that many bird species use single hemispheric sleepThis often allows them to take a nap during the flight. However, birds and dolphins use this half-sleep technique for different purposes. For example, in a flock of birds, most outsiders keep their eyes away from the flock to watch for predators. But dolphins do the opposite: When they sleep, they look at the rest of the pod, keeping one eye open to avoid being separated.

It is very difficult for scientists to measure the brain activity of animals that they cannot catch in the open sea. For example, sperm whale, blue whale or humpback whale. In this case, behavioral recordings remain the only indicator of sleep behavior. In such cases, researchers can attach tags to animals to monitor their behavior.

A study conducted by Miller in 2008 used special suction cup tags attached to sperm whales to monitor them as they slept briefly in the open ocean. It turns out that whales make shallow dives below the surface, take a vertical position, slow down their swimming speed to an almost complete stop, and slowly rise up. Their upward movement is probably due to the light oil from the water on their heads.

During sleep, entire pods of whales can be seen with their noses up, just below the ocean surface. At this time, the animals are completely unresponsive, indicating that they are in deep sleep. But, animals can only rest underwater for about 20 minutes, before they have to resurface to breathe air. After exhaling, the whale dives below the surface to rest and may continue this behavior. up to 3.5 hours.

How whales sleep: video

Northern elephant seals (Mirounga angustirostris) are also bihemispheric sleepers and take similarly short naps. A 2023 study led by Jessica Kendall-Barr, a postdoctoral researcher at the Scripps Institution of Oceanography at the University of California, San Diego, was the first to track the brain activity of a sleeping mammal at sea. Kendall-Barr and her colleagues found that seals dive to a depth of about 300 meters, at which point their brains slow down and enter REM sleep. While in REM sleep, seals turn upside down and slowly circle around in circles as they continue to sleep.

Seals sleeping in the ocean were upside down 100% of the time during REM sleep; This shows that they are paralyzed during REM sleep, just like us.
– Kendall-Bar explained in his research.

Possibly due to the risk of encountering a predator, seals limit their total sleep time at sea to two hours per day; This rivals the record for least sleep among all mammals, currently held by African elephants (about 2 hours per day).