Unihemispheric sleep is a special adaptation that allows animals to sleep with their eyes open. Basically, what happens is that one half of their brain sleeps, while the other half of their brain stays awake! This allows animals to get the rest they need while also staying alert to any dangers that may appear.
This special way of sleeping is actually more common in the animal kingdom than you may think. Here, we will take a look at some of the animals that are unihemispheric sleepers and therefore keep their eyes open when they sleep.
What animals sleep with their eyes open? There are many animals that sleep with their eyes open for various reasons. Some of the animals include, dolphins, whales such as the beluga whale, manatees, some bird species, but also fish and snakes.
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Marine Mammals
Marine mammals often sleep with their eyes open! For example, bottlenose dolphins sleep this way in order to get the much necessary sleep that they need while also making sure that they do not drown while they sleep.
This also occurs in whales such as the beluga whale. The half of the brain that is awake will allow the beluga whale to resurface for air when they need to breathe. Manatees (or sea cows) also have this ability. Just like dolphins and whales, manatees sleep with their eyes open in order to be able to come up for air. But manatees also need to keep their eyes open in order to keep a lookout for predators.
Manatees have to watch out for humans, sharks, killer whales, crocodiles, and alligators even while they sleep.
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Birds
Some birds also have the ability to sleep with their eyes open. Some of these birds may even be found in your own backyard!
Blackbirds are prey for a lot of other animals since they are so small. These birds sleep with their eyes open in order to get rest while also looking out for their predators (such as raccoons, weasels, snakes, skunks, foxes, and bigger birds).
Even birds that are not as small as blackbirds, like the peregrine falcon, will sleep with one eye open in order to make sure they do not become victims of their predators. Penguins will also sleep with their eyes open. Plus, penguins also usually sleep standing up! Both of these adaptations allow for penguins to be able to protect their young from various predators while sleeping.
Unihemispheric sleep is so important in birds such as ducks, because more than 50% of mallard ducks become prey during their first year of life, so it is important that these creatures are able to stay alert and on the lookout for predators while they sleep.
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Snakes and Fish
Snakes and fish are more examples of animals that sleep with their eyes open. But unlike the other animals above, they sleep with their eyes open for a different reason. That is because they do not have eyelids! Fish, ranging from goldfish to great white sharks, and snakes will sleep by staying almost completely still with their eyes completely opened and covered in transparent scales.
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Other Animals
Fruit bats appear to be the only non-marine mammal to do unihemispheric sleep. These bats will keep their eyes peeled for any threats while also sleeping. While most of these animals who sleep with their eyes open do so in order to ensure that they do not become victims, one animal, the South American sea lion, sleeps with their eyes open in order to make sure they never miss a meal!
These sea lions will use unihemispheric sleep to keep an eye out for any of their favorite foods – fish, fur seals, crustaceans, penguins, and squids. Additionally, scientists have recently observed crocodiles sleeping with one eye open. Crocodiles do not have very many natural predators, so scientists are unsure why they use unihemispheric sleep. Maybe they are like sea lions and they want to make sure they are not missing any potential meal go by!
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Why does my dog sleep with its eyes open?
It may seem funny to us, but some dogs will sleep with one or both of their eyes open. They do not do unihemispheric sleep though. Dogs are descended from wolves, so they still have some of the wild instincts in them.
Experts believe that some dogs sleep with their eyes partially open to give the illusion to predators that they are awake even when they aren’t (and even though, as domesticated pets, they are not wild animals that need to do this trick).