Siamese Crocodile (Photo Credit: By SuperJew CC BY-SA 3.0 via Wikimedia Commons)

While for humans the expression "sleep with one eye open" is just a metaphor to keep alert, for some animals it is a way of life. Called unihemispheric sleep, it is the ability to snooze with one eye open and the corresponding half of the brain awake, while the other half rests.

Researchers believe that this talent that has been observed in marine mammals like dolphins and seals, as well as numerous bird species serves many purposes. Birds use it to keep an eye out (literally) for predators while dolphins do it to keep track of their pods and young ones. Many social animals are also believed to use the technique to keep track of any interesting activities going on around them.

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Now researchers from Australia's La Trobe University have discovered that crocodiles may have also evolved this capability. While there has been plenty of anecdotal evidence about the reptiles 'sleeping' with one eye open', this is the first formal study that confirms the fact.

The scientists began by placing three juvenile saltwater crocodiles inside a large enclosure at the University's aquarium and filming their behavior. They noticed that when left undisturbed, the reptiles spent less than an hour with one eye open. The crocodiles instead seemed to prefer snoozing with both eyes shut just like the rest of us.

However, things changed drastically when the researchers introduced an external stimulus. They began by bringing a new young crocodile to the enclosure. Though it was one of their own, the other reptiles immediately appeared to perk up. The researchers noticed that the crocodiles slept with one open eye directly aimed at the newcomer, for longer amounts of time.

Siamese Crocodiles (Photo Credit: By SuperJew [CC BY-SA 3.0 via Wikimedia Commons)

The results were even more pronounced when a human was brought into the room. The reptiles kept one eye on the person and continued to look in the direction even when he/she was gone. Dr. John Lesku, who led the study says that the behavior mimics that of birds. He speculates that the reptiles evolved this capability from the common ancestor they shared with birds about 155 million years ago before the two split up to become entirely different species.

What's interesting is that it is not just the saltwater crocodiles that nap with one eye open. Lesku says colleagues from Germany's Max Planck Institute for Ornithology have observed the same behavior in adult Nile crocodiles and Caimans.

Unihemispheric sleep patterns (Image Credit:

The one thing the researchers are not sure of yet is if the reptiles are displaying typical unihemispheric sleep behavior. To verify that they will have to observe their brainwaves with the help of electroencephalogram (EEG) tests.

If the tests are positive, it would indicate that birds, mammals and reptiles all display this behavior. The only ones that don't? Land-based animals and humans that need to fully shut down the brain and eyes for a snooze. Lesku says that while we have always called the one-eyed napping animals odd, it may be that they are perfectly normal! This according to the researcher means that it is "our whole-brain sleep (that) becomes the evolutionary oddity!" Who would have thought?