We"ve all heard the expression "You"d better sleep with one eye open!" It"s usually uttered in connection to somebody being forewarned to beware of something unpleasant coming their way in the not-so-distant future. You can liken it to "Watch your back!" and other grim reminders. But like almost all expressions, there"s an origin for the sleeping-with-one-eye-open saying. That"s because certain animals and reptiles are known for doing it.

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Yes, many of our four- and two-legged friends have this particular trait and even some aquatic life practice it. In all likelihood, the habit was developed in order to survive in the wild. As the creatures have evolved over time, though, they have not lost touch with their ancient survival skills that got them where they are today.


Crocodile: Even these gigantic reptiles are suspected of exhibiting ASEC



Besides the very real occurrence of what is known as crocodile tears, a new study at La Trobe University in Australia has found the armored reptiles are also believed to practice the habit of unihemispheric sleep. While not much is known about the phenomenon in crocodiles, it is hypothesized that it is for the purpose of staying vigilant in case of attack. However, this seems unlikely as the sole reason of this documented sleep state, because crocs have almost no known predators other than man — and maybe bigger crocodiles.

It was also discovered that most of their sleeping hours were conducted with both eyes shut. Only when there was something to be aware of under controlled circumstances was it noted that they slept with one eye open. But were they really sleeping? Over time, as more studies are conducted (particularly brainwave) and more is unearthed regarding the behavior of these scary throwbacks from prehistoric times, scientists will have better theories for answering these questions.


Humboldt Penguins: South American penguins might be unihemispheric sleepers



According to BBC Earth, Humboldt penguins or Chilean penguins from South America are also reportedly on the list of unihemispheric sleepers. In their case, it could be presumed this condition is prompted by the need to stay semi awake in order to keep their eggs intact and then to keep their hatchlings in check. Both the eggs and the baby penguins are considered a culinary favorite of certain predators in the region. Unlike other penguins, the Humboldt penguin is known to be capable of reproducing at any time of the year, as long as food is plentiful.

Residing along the Pacific Coast, these penguins are found in both Chile and Peru and are said to enjoy warmer climates, as compared to other species of the flightless birds. It"s due to these warmer temperatures that they don’t engage in yearly migration rituals, basically staying put year-round. Another odd trait of slumbering penguins? For the most part, they stand up to sleep, especially in sub-zero climates.


Domesticated Barnyard Fowl: A variety of birds are known to be unihemispheric sleepers



Back in the late 1990s, it was determined certain birds sleep with one eye open as a defense mechanism as well. These are mostly birds that can fall prey to bigger birds and other animals or reptiles prone to eating anything within the proper size range. Some of those birds purportedly include geese, mallard ducks and domestic chickens, to name a few. Niels Rattenborg, a biologist at Indiana State University, believes birds can snooze with both eyes shut and both halves of their brain asleep, or they can rest half of their brain by utilizing the ASEC technique.

The habit of sleeping with one eye open isn"t exclusive to birds that sleep alone, either. It"s been reported in birds sleeping at the outer edges of flocks, whereas the birds more safely sequestered towards the center of the group tend to zonk out more fully with both eyes shut. "We have found that birds sleeping under risky situations spend more time with one eye open and half the brain awake, and choose to direct the open eye towards a perceived threat," said Dr. Rattenborg.


Manatees: Often called sea cows, these gentle creatures have more to fear from man


Animal & Human Sleep Studies

While some war veterans claim to have slept with one eye open during combat experiences that included extreme physical and emotional stress, it has never been definitively proven. In order to do so, scientists would have to replicate stress levels that would be off the charts. This isn"t really a great idea just to find out if somebody can really do it.

As far as animals, it"s been reported for many, many years now that hares or rabbits supposedly share the ASEC trait. But this is not surprising, if true, due to the fact they are prey animals and not predators. It will be interesting, however, to see what other species are capable of invoking this defense mechanism at will, as studies progress on the subject.

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Maybe someone can eventually tell us if house cats often sleep with their eyes at half-mast because, like their bigger cousins, they"re always on the lookout for their next meal. It seems you"re either the hunted of the hunter in this dog-eat-dog world, and it"s a real jungle out there.