Extreme Sleep: Wild Animals Doze in Perilous Environments

Sleep is universal across the animal kingdom, but where and how animals sleep can be as dramatic as the hazards they face. In a growing field researchers call extreme sleep, scientists are learning that wild species have evolved remarkably flexible naps to balance rest with predator risk and demanding tasks. For years, researchers could only guess when animals slept by watching for stillness; now tiny brain-wave trackers and miniature sleep helmets are giving scientists real-time glimpses into wild slumber.

One studied group is chinstrap penguins in Antarctica. Researchers tracked 14 adults over 11 days on King George Island and found these birds accumulate about 11 hours of sleep per day, despite their duties as full-time caregivers during the breeding season. They take thousands of micro-sleeps, each averaging around four seconds, which add up to sufficient rest while still staying alert for their eggs or chicks. The data show either hemisphere of the brain—or both—can be asleep, a pattern that lets penguin parents watch for predators while keeping an eye on their offspring.

Seabirds like great frigatebirds in the Galapagos illustrate another extreme: during long flights they can sleep with one hemisphere at a time while the other half remains semialert to monitor for obstacles. In-flight sleep allows them to soar for weeks at a time, riding warm updrafts rather than flapping. When back on land or in trees, frigatebirds tend to sleep with full-brain naps of longer bouts. Researchers say in-flight sleep is an adaptation that supports extended flying, though the birds cannot perform delicate maneuvers while partially asleep.

Dolphins and some other birds also show hemispheric sleep; dolphins have been shown to sleep with one hemisphere while swimming. Some swifts and albatrosses can sleep in flight. Frigatebirds have been documented sleeping 255 miles (410 kilometers) a day for more than 40 days at a stretch, a feat that would be impossible without the ability to rest on the wing.

On land, the northern elephant seal embodies another extreme. These 5,000-pound predators spend eight months foraging at sea, repeatedly diving to several hundred feet to chase fish, squid and rays. Each deep dive can last around 30 minutes, and during roughly a third of that time the seals sleep. In a 24-hour period, seals at sea sleep about two hours, compared with around 10 hours on beaches. Researchers led by Jessica Kendall-Bar of the Scripps Institution of Oceanography devised neoprene headcaps to detect motion and measure brain activity during dives, retrieving the caps when the seals returned to shore in Northern California. The 13 female seals studied tended to sleep during the deepest portions of their dives, when predators typically patrol those depths. That sleep included both slow-wave sleep and REM sleep, and during REM the seals’ dive motion sometimes changed into a “sleep spiral” as they briefly lost full motor control.

Scientists are still learning why sleep takes so many forms, and how much rest different species truly require. It’s clear, though, that sleep is remarkably flexible in response to ecological demands. “We’re finding that sleep is really flexible in response to ecological demands,” said Paul-Antoine Libourel, a researcher at the Neuroscience Research Center of Lyon in France. “Nature has evolved to make shut-eye possible in even the most precarious situations.”

The emerging field of extreme sleep shows that, while humans cannot replicate these feats, studying wild napping helps illuminate the fundamental purpose and adaptability of sleep across life on Earth.