Penguins in microsleep

Sleeping penguin

This chinstrap penguin on subantarctic King George’s Island is taking a short nap. © Won Young Lee

Sleep is vital for both us humans and animals. However, depending on the environmental conditions, periods of inattention are associated with significant risks. Researchers have now discovered an astonishing adaptation in chinstrap penguins in Antarctica: During the breeding season, the animals only sleep for a few seconds at a time, but thousands of times a day. In this way, they can sleep for a total of more than eleven hours a day without having to compromise on the protection of their clutches. The repeated microsleep seems to offer them enough rest.

If we don't sleep for a long time, we may unintentionally nod off for a moment. This so-called microsleep, in which our brain briefly switches to a sleep state, is particularly dangerous when driving. “But even in situations in which microsleep does not pose a direct danger, it is unclear whether it is long enough to confer any benefit from real sleep,” writes a team led by Paul-Antoine Libourel from the Neuroscience Research Center in Lyon in France.

How wild penguins sleep

To answer this question, Libourel and his team observed wild chinstrap penguins on King George Island in Antarctica. “During the breeding season, one penguin parent has to constantly guard the eggs and later the small chicks, while the other parent is out searching for food, sometimes for several days,” describe the researchers. “The penguins are therefore faced with the challenge of how they can sleep and protect their offspring at the same time.” The clutches are threatened primarily by skuas, which prefer to attack and plunder the nests on the edges of a penguin colony. Your own conspecifics can also become a problem because, if given the opportunity, they would steal nest-building materials that have been painstakingly collected.

For their study, Libourel and his team equipped 14 wild penguins with a series of measuring devices and sensors. In this way, the researchers were able to monitor the animals' brain waves, measure the muscle tension of their neck muscles, record their movements and determine their position. In some cases, they also recorded the breeding penguins on video. “During the observation phase, the penguins showed normal breeding behavior, alternating with the other parent on average every 22 hours,” reports the team. “In some cases, the other parent was away for more than 64 hours.”

Thousands of naps a day

Using the mobile EEG electrodes, the researchers were able to see when a penguin's brain showed the waves typical of sleep. It was already known from previous studies that penguins sometimes only sleep with one half of their brain, while the other half of the brain remains active and the corresponding eye is open. Libourel and his team were also able to confirm this in the current study. But regardless of whether they used both brain hemispheres or just one, the penguins' sleep phases only lasted an average of four seconds. Only 28 percent of the naps were longer than ten seconds, the longest being 34 seconds.

The penguins dozed off thousands of times a day. “Despite the shortness of the sleep phases, they accumulated a total of almost 15 hours per day in which they slept with one or both hemispheres of the brain,” report the researchers. “Each hemisphere of the brain received between 11.5 and twelve hours of sleep per day.” According to the results, sleep was particularly fragmented for the parent who stayed with the nest. The hunting parent, on the other hand, hardly slept during the dives and instead rested longer on land afterwards.

Disturbed sleep in the middle of the colony

The researchers also examined whether the position of the nest within the colony had an influence on the penguins' sleep quality. The offspring's most important predator, the brown skua, prefers to attack clutches on the outskirts of the colony. The team therefore assumed that the penguins could afford longer periods of sleep with nests in the middle of the colony. But the opposite was the case: Although their nest was better protected from skuas, these penguin parents slept even more fragmentedly than their counterparts further to the edge of the colony.

From the researchers' perspective, there are various possible explanations for this. One could be that the penguins on the edge of the colony have to be more alert during waking periods due to greater danger and therefore need more rest to compensate. However, it would also be conceivable that the conspecifics themselves represent a greater source of disturbance than the predators. On the one hand, the noise level inside the colony is very high, which can even disrupt microsleep. On the other hand, it is known that the penguins, if they have the opportunity, steal the small round stones from which the nests are built from each other.

Implications for sleep in general

But regardless of what exactly led the penguins to their unusual sleep pattern, there is an important insight for sleep in general: “The data reported may be one of the most extreme examples of how the benefits of sleep can also occur when sleep does not occur in one go,” write Christian Harding from the University of California at San Diego and Vladyslav Vyazovskiy from the University of Oxford in a commentary on the study, which was also published in the journal Science. “The constant switching between sleep and wakefulness in chinstrap penguins has important implications for understanding sleep regulation in general.”

However, this does not mean that the results can be directly transferred to humans. “What may be completely normal in birds or other animals may be abnormal or pathological in humans,” said Harding and Vyazovskiy. For example, fragmented sleep is known to be linked to neurodegenerative diseases in humans.

Source: Paul-Antoine Libourel (Neuroscience Research Center of Lyon, Bron, France) et al., Science, doi: 10.1126/science.adh0771

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