Extreme sleep adapts to perilous environments, from penguin colonies to aloft seabirds

Sleep is universal among animals, even in environments that put sleepers at risk. In the emerging field of extreme sleep, researchers are documenting how brains adapt rest to ecological demands—whether during non-stop parental duties, months of migration or weeks at sea. “Sleep is universal, even though it’s actually very risky,” said Paul-Antoine Libourel, a researcher at the Neuroscience Research Center of Lyon in France. Scientists say the flexibility of sleep becomes most evident when animals confront environments that tempt predators, harsh weather or the need to stay vigilant for offspring.

Chinstrap penguins in Antarctica offer one striking example. These birds mate for life and share parenting duties, with one parent watching the egg or chick while the other forages. To keep up this demanding routine for weeks, penguin parents rely on thousands of micro-naps that average about four seconds each. By measuring brain activity in 14 adults over 11 days on King George Island, researchers found that a penguin can sleep for roughly 11 hours in a 24-hour period, even as one hemisphere or the whole brain alternates between sleep and wakefulness. The naps are short enough to allow the caregiver to blink, assess nearby activity and react to a clumsy neighbor or a looming predator, then drift off again. The behavior demonstrates how sleep can be fractured yet functional in a crowded, noisy colony.

In flight, other birds push the limits of sleep as a way to sustain weeks of travel without touchdown. Great frigatebirds, nesting in the Galápagos Islands, exemplify one-half-brain sleep during long journeys. When gliding on thermal updrafts, these seabirds can let one hemisphere sleep while the other remains semialert, keeping an eye out for obstacles. They cannot perform complex maneuvers, such as flapping, foraging or diving, with only half a brain. Yet this unihemispheric slumber is enough to sustain hundreds of miles of travel for extended periods. Conversely, when they return to land or nest, frigatebirds switch to longer bouts of whole-brain sleep. Researchers note that in-flight sleep appears to be a specialized adaptation for extended flight, while in-nest sleep reflects different ecological needs. The species has been observed flying about 255 miles (410 kilometers) a day for more than 40 days at a stretch before reaching land or water again, a feat that would be unlikely without the ability to sleep on the wing.

Other large seabirds show similar hacks. Dolphins are well known for their unihemispheric sleep while swimming, allowing movement and surface breathing while one half of the brain rests. Additional birds, including swifts and albatrosses, have also been implicated in in-flight sleep, underscoring a broader pattern of sleep flexibility among flying species.

On land, the sleep story shifts to marine mammals that live most of their lives at sea. Northern elephant seals appear invulnerable on land but face a different risk while at sea: predators such as sharks and killer whales. During eight-month foraging trips, these 5,000-pound behemoths dive repeatedly to depths of several hundred feet in search of fish, squid and other prey. Researchers tracked 13 female seals with neoprene headcaps that logged motion and brain activity during dives. They found that about a third of their daily sleep occurs during the deepest parts of their dives, when predators are less likely to patrol. The sleep manifested as both slow-wave sleep and REM sleep. During REM, the seals’ dive motions could shift from controlled descents to upside-down spins described by scientists as a “sleep spiral.” Across a 24-hour cycle, the seals slept about two hours while at sea, compared with roughly 10 hours on land.

The studies, conducted with motion sensors and brain-wave measurements, reflect a broader shift in how scientists study sleep in the wild. Tiny trackers and helmet-like devices—miniaturized versions of human sleep lab equipment—have enabled researchers to observe when and how wild animals snooze without removing them from their natural environments. In the Antarctic, the King George Island study followed 14 adult penguins for 11 days, while tracking devices on frigatebirds in the Pacific and headcaps on elephant seals yielded complementary insights into how animals balance rest with ecological demands.

The researchers, including Niels Rattenborg of the Max Planck Institute for Biological Intelligence in Germany, emphasize that what looks like sleep on land may be a strategic compromise underwater or in the air. “We’re finding that sleep is really flexible in response to ecological demands,” Rattenborg said. Libourel and colleagues argue that the growing field—sometimes called extreme sleep—reveals a spectrum of strategies that make shut-eye possible in otherwise perilous situations.

The evolving picture of sleep in the wild underscores the complexity of sleep’s function. Scientists continue to refine tools and methods to determine when and how animals rest and what triggers transitions between waking and sleep states. While these natural hacks are unlikely to be replicated by tired humans, they illuminate how evolution shapes core physiological needs to fit ecological realities.

The research builds on years of work that began with the recognition that animals often appear still and quiet, leading early researchers to infer sleep from behavior alone. The advent of brain-wave monitoring in free-ranging wildlife has allowed scientists to confirm that rest can occur in isolated bursts or during single-hemisphere states, and that such sleep patterns are intimately tied to a species’ life history and environment. As tracking technology improves, researchers expect to map additional examples of extreme sleep, further clarifying why sleep persists as a vital, yet pliable, process across the animal kingdom.