Every animal with a brain needs sleep—even some animals without brains need sleep. Humans sleep, birds sleep, whales sleep, even jellyfish sleep.
Paul-Antoine Libourel, a researcher at the Center for Neuroscience Research in Lyon, France, said sleep is ubiquitous “even though it is actually very dangerous.”
When animals doze, they are most vulnerable to attacks from cunning predators. Despite the risks, the need for sleep is so strong that no living thing can skip it entirely, even if it is extremely inconvenient.
Animals that survive in extreme conditions and environments have evolved to sleep in extreme ways—for example, stealing a few seconds at a time during round-the-clock parenting, blinking their wings during long-distance migrations, or even dozing off while swimming.
For a long time, scientists could only make educated guesses about when wild animals slept, observing when they stood still and closed their eyes. But in recent years, tiny trackers and helmets that measure brain waves — miniature versions of human sleep lab equipment — have given researchers their first look at the various, sometimes spectacular, ways wild animals nap.
“We found that sleep is very flexible in response to ecological needs,” said Niels Rattenborg, an expert on animal sleep at the Max Planck Institute for Biointelligence in Germany.
The emerging science of calling it “extreme sleep.”
Chinstrap penguins and their “microsleep”
Take, for example, the chinstrap penguins in Antarctica that Liebrer studied.
These penguins mate for life and share parenting responsibilities – one bird takes care of the egg, or gray fluffy chick, to keep it warm and safe, while the other swims off to fish and provide a meal for the family. Then they switch roles—a non-stop labor that lasts for weeks.
Penguin parents face a common challenge: getting enough sleep while paying close attention to their newborns.
They take thousands of naps a day to survive – each nap lasts an average of just 4 seconds.
Won Young Lee, a biologist at the Korea Polar Research Institute, said these brief “microsleeps” appear to be enough for penguin parents to perform caregiving duties in crowded, noisy habitats for weeks.
When an awkward neighbor passes by or a predatory seabird approaches, a penguin parent blinks for attention, then quickly dozes off again, chin resting on his chest, like a drowsy driver.
Naps add up. By measuring the brain activity of 14 adult penguins on King George Island in Antarctica for 11 days, scientists found that each penguin slept a total of 11 hours a day.
To remain mostly alert while also furtively blinking, penguins have evolved an enviable ability to function in extremely fragmented sleep—at least during breeding season.
Researchers can now see when either hemisphere of the brain, or both hemispheres at the same time, is asleep.
Frigatebirds doze off with half their brains while flying
Poets, sailors and birdwatchers have long wondered whether birds that fly for months at a time actually blink.
In some cases, the answer is yes—as scientists found when they attached equipment that measured brain wave activity to the heads of large seabirds nesting in the Galapagos Islands, called great frigatebirds.
While flying, frigatebirds can sleep using half their brains at a time. The other half remains semi-alert, with one eye still watching for obstacles in the flight path.
This allows the birds to soar for weeks at a time without coming into contact with land or water that would damage their fragile, non-waterproof feathers.
Frigatebirds can’t perform tricky maneuvers – flapping their wings, foraging or diving – using only half their brains. They must be fully conscious when they dive into the water in search of prey. But in flight, they have evolved to sleep while gliding and spiraling upward in huge warm updrafts, allowing them to stay aloft with minimal effort.
After returning to their nests in trees or bushes, frigatebirds changed their napping habits—they were more likely to use their entire brains to sleep at once, and for much longer periods of time. Rattenberg said this suggests that their in-flight sleep is a special adaptation for long flights.
Some other animals have similar sleep techniques. Dolphins can sleep with half their brains at a time while swimming. Scientists say some other birds, including swifts and albatrosses, can sleep in flight.
Other researchers have found that frigatebirds can fly 255 miles (410 kilometers) per day for more than 40 days before landing, a feat that would be impossible without sleeping on their wings.
Elephant seal snoozes during deep dive
On land, life is easy for the 5,000-pound (2,268-kilogram) northern elephant seal. But at sea, sleep can be dangerous—seal-eating sharks and killer whales lurk.
The seals go on long foraging trips, lasting up to eight months, repeatedly diving to depths of hundreds of feet (meters) to catch fish, squid, rays and other ocean snacks.
Each deep dive may last approximately 30 minutes. Research led by Jessica Kendall-Barr of the Scripps Institution of Oceanography shows that seals may be sleeping about a third of the time.
Kendall-Barr’s team designed a neoprene headgear similar to a swimming cap equipped with equipment to detect movement and brain activity of the seals during dives, and retrieved the cap with the recorded data when the seals returned to Northern California beaches.
The 13 female seals in the study tended to sleep at their deepest dives, when they were below depths where predators typically patrol.
This type of sleep includes slow wave sleep and rapid eye movement sleep. During REM sleep, seals become temporarily paralyzed—much like humans do during deep sleep stages—and their diving movements change. During REM sleep, they sometimes turn upside down and rotate in what researchers call a “sleep spiral,” rather than a controlled downward sliding motion.
Over the course of 24 hours, the seals slept for a total of about two hours. (After returning to the beach, they spent an average of about 10 hours.)
The twists and turns of sleep
Scientists are still learning about all the reasons why we sleep, and how much sleep we really need.
No exhausted human being would ever attempt these extreme animal sleep techniques. But learning more about the diversity of naps in the wild shows the flexibility of some species. Nature has evolved to close its eyes even in the most dangerous situations.
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The Associated Press Health & Science Department receives support from the Howard Hughes Medical Institute’s Department of Science Education and the Robert Wood Johnson Foundation. The Associated Press is solely responsible for all content.