Crocodiles Keep One Eye Open As They Sleep, New Study Shows

When animals close their eyes and to rest, they inevitably become more susceptible to predation. To remain in defense mode, many have developed a specialized sleeping system that allows them to keep half their brain alert, while the other half sleeps. This is called unihemispheric sleep – what's commonly known as "sleeping with one eye open." Crocodiles, in particular, have a long-held reputation for sleeping with one eye open. 

To find out if the age-old belief is true, researchers from Australia's La Trobe University and Germany's Max Planck Institute for Ornithology observed the "eye states" of juvenile saltwater crocodiles (Crocodylus porosus) that were kept in a special holding aquarium. They set up cameras that recorded the crocodiles in action for 24-hours at a time. Then they sent a crocodile or a human into the holding cell to see how these reptiles would respond to sleep disruptions, according to the group's news release. 

While the crocodiles did not appear to rest with one eye open as a rule, researchers found that the animals were more likely to open an eye to keep track of the intruder. The crocodiles took extra caution with human intruders, specifically fixing their gaze on them and tracing their every move. 

"These findings are really exciting as they are the first of their kind involving crocodilians and may change the way we consider the evolution of sleep," Michael Kelly, lead researcher from La Trobe University, said in the release. "What we think of as 'normal' sleep may be more novel than we think."

Further research is required to determine if crocodiles are actually – and technically – still asleep when that single eye slides open (or even to find out why its just one eye and not both). To do so, researchers would need to measure the reptilian's brain waves, Lesku explained. Crocodiles are among some of the earliest reptiles to have ever lived and if they're actually engaging in unihemispheric sleep, continued studies could help researchers better understand how the process originated. 

"Some birds and aquatic mammals sleep unihemispherically with one eye open. If ultimately crocodilians and other reptiles that have been observed with only one eye closed are likewise sleeping unihemispherically then our whole-brain (or bihemispheric) sleep becomes the evolutionary oddity," Dr. John Lesku, a professor from La Trobe's School of Life Sciences, added.

Unlike crocodiles, humans shut off their brains completely when they sleep.

This study has provided researchers with a better understanding of species' differing sleep cycles and their effectiveness. The findings were published in the Journal of Experimental Biology. 

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