Chinstrap penguins are one of nature's most extreme nappers, snatching more than 10,000 microsleeps lasting up to four seconds long every day, according to a new study.

The penguins, which live in breeding colonies each containing tens of thousands of individuals, must remain constantly vigilant against threats to their nests from predatory skua birds and hostile colony neighbors.

The result is a sleeping schedule warped beyond all recognition — placing the penguins in a state somewhere between wakefulness and sleep that culminates in 11 hours of snoozing time per day. The researchers behind the study published their findings Dec. 30 in the journal Science.

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"It shows nicely at what point sleep is constrained by natural selection processes," Paul-Antoine Libourel, a researcher at the Lyon Center for Research in Neuroscience in France, told Live Science. "Animals face a clear tradeoff between sleeping and its benefits while not being vigilant, and being awake but with the physiological cost of not sleeping. These penguins found a way to gain the advantage of sleeping while also remaining vigilant to protect their eggs."

Named after the thin black strip that runs under their beaks, chinstrap penguins (Pygoscelis antarcticus) live on islands all the way around the South Pole. During the nesting season, males squat on eggs to provide protection and warmth while females leave on long hunts. In the chaotic and noisy environment of the colony, any sleep is welcome — but it comes at a cost of decreased watchfulness.

To study how the penguins managed the careful balancing act, the researchers studied the sleep patterns of 14 penguins out of a colony of 2,700 breeding pairs on King George's Island off the coast of Antarctica. After surgically implanting electrodes into the penguins' brains and connecting them to data loggers placed on their backs, the researchers released them back onto the island.

After weeks of watching the penguins nodding off, the scientists had conclusive evidence of one of nature's most bizarre zonk strategies: the nesting penguins take 600 microsleeps an hour, each lasting an average of four seconds. By monitoring the brain activity of the drowsy birds as they microdozed, the researchers also found that sometimes they only slept with half of their brain, while the other hemisphere remained alert.

And, to the researchers' surprise, all of these tiny sessions added up, providing a restorative function to the brains of the penguins across the entire day.

Microsleep has been spotted in other birds and marine mammals, including albatrosses, dolphins, ducks and elephant seals — an adaptation that enables them to rest while on the move — but never for such extended periods of time, Libourel said.

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"There are reports of drowsy states in other species, looking like microsleeps," he said. "However, those studies do not report any animal always sleeping in this way."

The scientists say that the chinstrap penguins' unusual dozing challenges many dominant assumptions about sleep across the animal kingdom. Their next steps will be to look for more animals with weird sleeping patterns.

"The data reported by Libourel et al. could be one of the most extreme examples of the incremental nature by which the benefits of sleep can accrue," Vladyslav Vyazovskiy, a professor of sleep physiology, and Christian Harding, a sleep researcher, both at the University of Oxford, wrote in a related commentary article on the discovery.

"Proving that sleeping in this way comes at no cost to the penguin would challenge the current interpretation of fragmentation as inherently detrimental to sleep quality," they wrote.