Every animal with a brain needs sleep, and even some animals that don't have a brain need it too. People are sleepingBirds sleep, whales sleep, and even jellyfish sleep.
Sleep is universal, “although it is actually very risky,” said Paul-Antoine Libourel, a researcher at the Neuroscience Research Center in Lyon, France.
When animals are dozing, they are most vulnerable to sneaky predators. But despite the risks, need for sleep so powerful that no creature can miss it at all, even if it is extremely inconvenient.
Animals that navigate extreme conditions and environments have evolved to sleep in extreme ways, such as stealing. seconds at a time during round-the-clock parenting, winking on the fly during long migrations, and even dozing while swimming.
For a long time, scientists could only speculate about when wild animals sleep by observing when they lie still and close their eyes. But in recent years, tiny trackers and helmets that measure brain waves (miniature versions of the equipment in human sleep laboratories) have allowed researchers to see for the first time the varied and sometimes spectacular ways in which wild animals doze.
“We are finding that sleep is really flexible in response to environmental demands,” said Niels Rattenborg, an animal sleep researcher at the Max Planck Institute for Biological Intelligence in Germany.
Call it the emerging science of “extreme sleep.”
Take chinned penguins in Antarctica, which Libourel studies.
These penguins mate for life and share parental duties: one bird nurses the egg or tiny gray fluffy chick to keep it warm and safe, while the other swims off to catch fish for the family dinner. They then switch roles, continuing this continuous work for weeks.
Penguin parents face a common challenge: getting enough sleep while keeping a close eye on their newborns.
They survive by taking thousands of naps a day, each nap lasting only 4 seconds on average.
These short “micro-sleeps,” as Korean Polar Research Institute biologist Won Young Lee calls them, are apparently enough to allow penguin parents to perform their caregiving duties for several weeks in their crowded and noisy colonies.
When a clumsy neighbor walks by or predatory seabirds are nearby, the parent penguin blinks to attract attention and soon falls back asleep, nodding its chin to its chest like a sleepy driver.
Sleep adds up. Each penguin sleeps a total of 11 hours a day, scientists found by measuring the brain activity of 14 adults over 11 days on King George Island in Antarctica.
To remain alert and wink frequently enough at the same time, penguins have developed an enviable ability to function in extremely intermittent sleep conditions—at least during the breeding season.
Researchers can now see when one hemisphere of the brain—or both—is asleep.
Poets, sailors and ornithologists have long wondered whether birds that fly for months actually have a wink on their wing.
In some cases, the answer is yes, as scientists discovered when they attached devices that measure brain wave activity to the heads of large seabirds that nest in the Galapagos Islands, called great frigatebirds.
During flight, frigate birds can sleep with one half of their brain at a time. The other half remains semi-vigilant, so that one eye is still watching for obstacles in the flight path.
This allows the birds to soar for weeks without touching the ground or water, which could damage their delicate, non-water-repellent feathers.
Frigate birds cannot perform complex maneuvers—flapping their wings, searching for food, or diving—using only one half of their brain. When they dive for prey, they must be fully awake. But in flight, they have adapted to sleep as they glide and circle upward on huge currents of warm, rising air that keep them aloft with minimal effort.
Once back at their nest in trees or bushes, frigatebirds change their sleep patterns—they are more likely to sleep whole-brain at once and for much longer periods. This suggests that their in-flight sleep is a special adaptation for long flights, Rattenborg said.
Some other animals have similar sleep tricks. While swimming, dolphins can sleep on one side of their brain at a time. Some other birds, including swifts and albatrosses, can sleep in flight, scientists say.
Other researchers have found that frigatebirds can fly 255 miles (410 kilometers) a day for more than 40 days before touching land—a feat that would be impossible without the ability to sleep on the wing.
On land, life is easy for the 5,000-pound (2,268-kilogram) northern elephant seal. But in the sea, sleep is dangerous – sharks and killer whales lie in wait and hunt seals.
These seals undertake long foraging trips lasting up to eight months, repeatedly diving to depths of several hundred feet (meters) to catch fish, squid, rays and other marine snacks.
Each deep dive can last about 30 minutes. And about a third of that time, seals may be sleeping, according to research led by Jessica Kendall-Bahr of the Scripps Institution of Oceanography.
The Kendall-Bar team developed a neoprene cap, similar to a swim cap, with equipment to detect the movement and brain activity of the seals during dives, and retrieved caps with recorded data when the seals returned to beaches in Northern California.
The 13 female seals studied tended to sleep during the deepest periods of diving, when they were below the depths that predators typically patrol.
This sleep consisted of both slow wave sleep and REM sleep. During REM, or rapid eye movement, sleep, the seals were temporarily paralyzed—just like humans during this stage of deep sleep—and their diving movements changed. Instead of moving in a controlled downward manner, they sometimes flipped upside down and rotated, in what the researchers called a “sleep spiral” during REM sleep.
Over a 24-hour period, the seals slept for a total of about two hours at sea. (Back at the beach, they spent an average of about 10 hours.)
Scientists are still studying all the reasons why we sleep – and only how much do we really need.
It is unlikely that a tired person will be able to try these extreme life hacks for animal sleep. But studying how varied sleep can be in the wild shows the flexibility of some species. Nature has evolved to make it possible to close the eyes even in the most dangerous situations.
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