Nodding off is dangerous. Some animals have evolved extreme ways to sleep in precarious environments

All animals with brains need sleep – and even some without brains need it too. Humans sleep, birds sleep, whales sleep, and even jellyfish sleep.

Sleep is universal “even though it’s actually very risky,” said Paul-Antoine Libourel, a researcher at the Lyon Neuroscience Research Center in France.

When animals fall asleep, they are more vulnerable to sneaky predators. But despite the risks, the need for sleep is so strong that no creature can do without it completely, even when it is very inconvenient.

Animals that thrive in extreme conditions and environments have evolved to sleep in extreme ways – for example, flying for seconds at a time during around-the-clock parenting, receiving winks on the wing during long migrations, and even dozing while swimming.

For a long time, scientists could only make educated guesses about when wild animals slept, by observing when they stayed still and closed their eyes. But in recent years, tiny trackers and headsets measuring brain waves – miniaturized versions of equipment found in human sleep laboratories – have allowed researchers to glimpse for the first time the varied and sometimes spectacular ways in which wild animals sleep.

“We find that sleep is really flexible in response to ecological demands,” said Niels Rattenborg, a specialist in animal sleep research at the Max Planck Institute for Biological Intelligence in Germany.

Let’s call this the emerging science of “extreme sleep.”

Chinstrap penguins and their “microsleeps”

Take the chinstrap penguins in Antarctica that Libourel studies.

These penguins mate for life and share parental duties: one bird guards the fluffy gray egg or little chick to keep it warm and safe while the other swims off to fish for a family meal. Then they switch roles – continuing this incessant work for weeks.

Penguin parents face a common challenge: getting enough sleep while keeping a watchful eye on their newborns.

They survive by taking thousands of naps a day, each lasting on average just 4 seconds.

These short “microsleeps,” as Won Young Lee, a biologist at the Korea Polar Research Institute, calls them, appear to be enough to allow penguin parents to carry out their caregiving duties for weeks in their crowded, noisy colonies.

When a clumsy neighbor passes by or predatory seabirds are nearby, the penguin parent blinks for attention and soon dozes off again, its chin nodding against its chest, like a drowsy driver.

Naps add 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, Antarctica.

To stay generally alert, while still winking sufficiently, penguins have evolved an enviable ability to function on extremely fragmented sleep – at least during the breeding season.

Researchers can now see when one or the other hemisphere of the brain – or both – is asleep.

Frigate birds sleep half their brains in flight

Poets, sailors and birders have long wondered whether birds that fly for months at a time actually wink in flight.

In some cases, the answer is yes, as scientists discovered by attaching devices measuring brain wave activity to the heads of large seabirds nesting in the Galapagos Islands, called great frigatebirds.

In flight, frigatebirds can sleep with half their brain at a time. The other half remains semi-alert, so one eye is always watching for obstacles in its flight path.

This allows the birds to glide for weeks without touching land or water, which would damage their delicate, non-water-repellent feathers.

Frigatebirds cannot perform delicate maneuvers – flapping their wings, foraging for food or diving – with only half their brains. When diving for prey, they must be fully awake. But in flight, they evolved to sleep as they hover and spin upward on massive, upward air currents that keep them aloft with minimal effort.

Returning to the nest in trees or bushes, frigate birds change their nap routine: they are more likely to sleep with all their brains at once and for much longer periods. This suggests that their in-flight sleep is a specific adaptation for extended flights, Rattenborg said.

A few other animals have similar sleeping habits. Dolphins can sleep with half their brain at a time when swimming. Some other birds, including swifts and albatrosses, can sleep in flight, scientists say.

The frigates can fly 260 miles a day for more than 40 days, before making landfall, other researchers have found – a feat that wouldn’t be possible without being able to sleep on the wing.

Elephant seals sleep while diving deep

On land, life is easy for a northern elephant seal weighing 5,000 pounds (2,268 kilograms). But at sea, sleeping is dangerous: sharks and killer whales that feed on seals hide.

These seals make 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 around 30 minutes. And for about a third of that time, the seals could be sleeping, research by Jessica Kendall-Bar of the Scripps Institution of Oceanography found.

The Kendall-Bar team designed a neoprene cap similar to a swimming cap, equipped with equipment to detect movement and seal brain activity during dives, and collected the caps with the data recorded when the seals returned to Northern California beaches.

The 13 female seals studied tended to sleep during the deeper parts of their dives, when they were below depths usually frequented by predators.

This sleep consisted of both slow-wave sleep and REM sleep. During REM sleep, or rapid eye movement sleep, the seals were temporarily paralyzed – just like humans during this deep sleep phase – and their diving motion changed. Instead of a controlled downward sliding motion, they sometimes tossed and turned in what researchers called a “sleep spiral” during REM sleep.

Over the course of 24 hours, the seals at sea slept for about two hours in total. (Back on the beach, they averaged about 10 hours.)

The winding evolution of sleep

Scientists are still learning more about all the reasons we sleep and how much we actually need.

It is unlikely that a tired human being would try these extreme animal sleeping tricks. But learning more about the diversity of naps in the wild shows the flexibility of some species. Nature has evolved to make it possible to close our eyes, even in the most precarious situations.

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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Department of Science Education and the Robert Wood Johnson Foundation. The AP is solely responsible for all content.