Chinstrap Penguins Sleep Over 10,000 Times a Day—for Just Four Seconds at a Time

The meeting drones on, and you feel your eyes getting heavy, your mind drowsy. Suddenly you snap into a very attentive wakefulness—did anyone see me doze off? You’ve fallen into a microsleep, a very brief slip out of consciousness that ends almost as soon as it begins. When you’re trying to be awake and active, these episodes may make you feel anxious or, if they happen behind the steering wheel, justifiably terrified.

Microsleeps don’t make us feel rested or restored, like longer periods of uninterrupted slumber would. Yet a study published Thursday in Science shows that nesting chinstrap penguins sleep just this way more than 10,000 times a day. They take a continual series of dozes that last just four seconds but add up to more than 11 hours of slumber. Incredibly, this strange sleep cycle seems to do the birds no obvious harm, despite the common interpretation that fragmented sleep is bad-quality. In fact, the extreme strategy must preserve at least some benefits of sleep, because the flock is fit and successfully reproduces.

“What is really weird is that the penguin can sustain this in-between wake and sleep state constantly,” explains co-author Paul-Antoine Libourel, who studies the biology of sleep at the French National Center for Scientific Research’s Neuroscience Research Center. Simply watching the penguins nod and blink gave the appearance that they were drowsy, he adds, but the extent of their sleeping was a surprise. “Only by constantly recording brain activity, for days, have we been able to highlight this interesting sleep phenotype.”

The extreme adaptation may be driven by environmental factors at King George Island, off Antarctica, where penguins flock together to incubate eggs and protect their young from predators. The need to sleep briefly could simply be a consequence of living in a bustling, noisy group where sleep is constantly interrupted. It might also help the birds remain constantly vigilant for predators. The penguins’ successful microsleep strategy raises interesting questions about how variable fruitful sleep can be among different species and in different environments. It also suggests that our bias toward the importance of longer, uninterrupted sleep may not be accurate—some species may also benefit from fragmented sleep.

“Sleep provides a lot of benefits, but we don’t know whether it’s the same benefits for all species,” Libourel says. “And we don’t know at what point we can disturb sleep, with or without cost to the animal.”

For the experiment, the scientists implanted electrodes into 14 penguins’ brain and neck muscles. For two weeks, they constantly recorded electroencephalogram (EEG) data to measure the birds’ sleep-related brain activity. Other sensors recorded the birds’ positions (standing, lying, diving); tracked their location via GPS; and captured environmental data like ambient temperature. The authors also deployed continuous video monitoring of the same nesting birds so that their observable behaviors could be matched with simultaneous data on their brain activity.

EEG recordings of brain activity showed when the birds engaged in slow-wave sleep, the most common type of sleep in birds. Here, researchers discovered that the birds nodded off thousands of times per day, engaging in microsleeps that averaged just four seconds each but added up to more than 11 hours of sleep per day.

As is normal among nesting penguins, paired parents took turns guarding and incubating the nests on land, and heading to sea on foraging trips—with each shift averaging about 22 hours. At sea, the birds did experience some slow-wave sleep while apparently resting at the water’s surface. However, they spent far more time awake and active on these forays. They were awake perhaps two-thirds of the time, but immediately upon returning to land, the birds spent the first few hours catching up on sleep along the shorelines. When the birds returned to lying or standing at the nests, they rested by shifting into the novel microsleep strategy.

Why would the birds adopt this fascinating sleep cycle? The constant vigilance may help keep the nests safe from predators like the brown skua bird, which feeds on penguin eggs and chicks. Snatching only seconds of sleep at a time would allow the birds to be remain relatively alert to any possible dangers.

Vladyslav Vyazovskiy, a sleep physiologist at the University of Oxford not affiliated with the research, theorizes that one might think of the colony as a kind of superorganism. Because each individual bird is constantly alternating between sleep and wakefulness, the entire group is at all times only half asleep, and also remains half awake. “I’ve been thinking a lot that we should not consider sleep as an individual phenomenon,” he says. Sleepers are influenced by the environment in which they doze, he explains. That can include the social environment: the actions of others within a group.

Interestingly, sleep patterns varied among birds based on their location. Penguins nesting on the fringes of the colony had longer, deeper sleeps than those in the middle. That may be because penguins in the center occupy noisy real estate. “There are more neighbors and passers-by in the center positions, so their sleeps are more likely to be interrupted,” suggests co-author Won Young Lee, a behavioral ecologist from the Korea Polar Research Institute.

The birds on the periphery must be more vigilant for predators, which exacts a cost. “The more intense your wakefulness, the more intense your sleep needs to be,” Vyazovskiy says. “Wake quality effects sleep quality. So maybe the birds on the periphery need to be extra vigilant, and they get tired, and then they sleep a bit longer and in a deeper way.”

Lee also wonders whether there are seasonal or other variations in the way the entire flock sleeps. “It would be interesting if we can estimate their sleep after breeding,” he notes. “I wonder if they continue microsleeps for their entire life or change sleep patterns with their breeding stages or life stages.”

Most sleep research takes place in controlled settings like laboratories. But as technology makes it more possible to study the brain in real-time, real-world environments, valuable new possibilities emerge. “We need to be open minded that sleep is so exquisitely sensitive to the environment, and therefore environment must be a very important part of the picture,” Libourel notes.

For example, a recent study among jackdaws explored seasonal sleep variation and found the birds sleep some five hours less during the summer than they do in the winter. Their sleep appears highly sensitive to environmental changes like shorter days. Nocturnal primates called Javan slow lorises sleep in long stretches like humans. But they also shift their sleep rhythms to match changing daylight hours and are sensitive to temperature changes, which disrupt their daytime sleep and prompt more frequent naps.

Many interesting types of sleep have evolved as traits unique to an animal’s lifestyle. Some birds, for example, can sleep on the wing, but Libourel wonders how flexible and adaptive sleep can be while still delivering essential benefits. How quickly can species adjust their sleeping habits when their environmental situations shift? That’s an important question in a world where wildlife is experiencing change at a very rapid rate.

As for the penguins, Christian Harding, an interdisciplinary bioscientist who focuses on sleep studies at the University of California, San Diego, health system, says their microsleep strategy may have evolved to provide some fitness advantages suited to life in Antarctica. It isn’t yet known whether such a cycle could also benefit other species. “Are chinstrap penguins simply the best at utilizing a strategy available to us all?” he asks.

For humans, at least, the prospect isn’t very appealing. While a short nap is often welcome, a constant succession of four-second sleeps sounds more like the stuff of nightmares.

Brian Handwerk | READ MORE

Brian Handwerk is a science correspondent based in Amherst, New Hampshire.