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11 Animals Known To Sleep With One Eye Open

Getting a good sleep is essential for animals as it helps them fully recharge and prepare for the next day. However, we found out that while most animals sleep with both their eyes closed, there are certain animals that sleep with one eye open. This unique sleep pattern, referred to as “unihemispheric sleep,” was first observed in 1964, and this enables some creatures to maintain a delicate balance between rest and mindful alertness by having half of the brain rest and the other half remain conscious.

This article will explore a few creatures that use this strategy to thrive in the environments they live in.

11 Animals that sleep with one eye open

1. Bottlenose Dolphins

Bottlenose Dolphin breaching
Bottlenose Dolphin breaching | image by caroline legg via Flickr | CC BY 2.0

Scientific Name: Tursiops truncatus

The bottlenose dolphin is a highly social aquatic mammal that you can find everywhere except the polar waters. You might also see these animals sleeping with one eye open, with the left eye closing when the right half of the brain sleeps, and the opposite happening when the other half sleeps. 

Because of this, these dolphins can maintain a level of awareness of their surroundings even while they’re sleeping. This also helps them to be conscious all the time and know when they need to go to the surface for air because bottlenose dolphins are aquatic mammals that need to breathe on the surface from time to time. 

2. Mallard Ducks

Mallard duck
Mallard duck

Scientific Name: Anas platyrhynchos

You might also come across another animal with this trait – the mallard duck, a species you can find in almost every corner of the world. After the breeding season, they often travel in large groups from the cold north to the warmer south. Due to the high number of predators they face in the wild, these ducks display unihemispheric slow-wave sleep, a condition in which one eye remains open as the other half of the brain rests.  

3. Beluga Whales

Beluga Whale underwater
Beluga Whale underwater | image by Scott via Flickr | CC BY-SA 2.0

Scientific Name: Delphinapterus leucas

Beluga whales, which are also known as white whales, have a distinctive feature that makes them easily recognizable: they have large melon-like structures on their foreheads. You can spot these whales peacefully floating on the water’s surface, resting with one of their eyes open. Just like dolphins, this ability helps them take in the needed amount of air even when sleeping. 

4. Saltwater Crocodiles

Saltwater crocodile on bank
Estuarine / Saltwater Crocodile | image by Djambalawa via Wikimedia Commons

Scientific Name: Crocodylus porosus

When you visit the coasts of northern Australia, you’ll have the opportunity to see the saltwater crocodile, one of the reptiles that inhabit this region. Even though they can survive in environments with a high salinity tolerance, they’re sometimes discovered in freshwater rivers, billabongs, and swamps. 

Saltwater crocodiles are the largest reptile alive today, reaching lengths of up to 23 ft and weighing around 1100 lbs. It was once just legend in Australia, but researchers have confirmed that saltwater crocodiles actually do sleep with one eye open. 

5. Emperor penguin

Emperor penguin with baby
Emperor penguin with baby | Image by Siggy Nowak from Pixabay

Scientific Name: Aptenodytes forsteri

Emperor penguins are extremely sociable birds that are capable of being active at any time of the day or night, depending on a variety of factors, including the accessibility of food.

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In addition, they have a polyphasic sleep pattern where they sleep for a total of 10 hours, but this sleep is divided into shorter increments ranging from 4 to 7.5 minutes each. Because they inhabit cold regions, emperor penguins choose to sleep while standing up so that as little of their body as possible comes into contact with the frozen ground. 

6. Amazonian manatees

Amazonian manatee on blue water
Amazonian manatee on blue water | image by Gerson Barreiros via Wikimedia Commons | CC BY-SA 3.0

Scientific Name: Trichechus inunguis

The Amazon manatee, another species that sleeps with an eye open, wakes up periodically throughout their sleep to breathe air from the water’s surface. Because of this, they require consciousness even when they’re at rest so they can occasionally emerge to breathe.

These animals can consume up to eight percent of their body weight in aquatic vegetation in a single day, so dense vegetation in blackwater lakes, oxbows, and lagoons is a common habitat for them. 

7. South American sea lions

South American sea lion
South American Sea Lion | image by Killy Ridols via Wikimedia Commons | CC BY-SA 2.0

Scientific Name: Otaria flavescens

Sea lions are marine mammals that are capable of sleeping at any time of the day or night, and it’s possible to find them dozing in or out of the water, depending on their environment. You can often find them in groups, along shorelines and beaches.

However, don’t be surprised if you also stumble upon them on flat rocky shelves or cliffs with tidepools and boulders. Resting with an eye open is beneficial for them because it helps them stay alert and aware of any potential predators in their surroundings. 

8. Wahlberg’s epauleted fruit bat 

Wahlberg’s epauleted fruit bat
Wahlberg’s epauleted fruit bat | image by Bernard DUPONT via Flickr | CC BY-SA 2.0

Scientific Name: Epomophorus wahlbergi

In the southern region of the Sahara desert, you can come across a bat species known as Wahlberg’s epauleted fruit bat. These creatures are easily recognizable by their brown coloration, with patches of white hair at the base of their ears. 

These bats may be the first non-marine mammals to show evidence of one-eye-sleep, which they do during the summer. Temperature also significantly impacts their sleeping time, as the more extreme the heat, the less time they spend sleeping and more time cooling themselves.

9. Geese

Graylag goose
Graylag goose | image by Imran Shah via Flickr | CC BY-SA 2.0

Scientific Name: Anser anser

The goose is one of the creatures you often see with one eye not closed when sleeping. Unlike other bird species, which sleep on their nests, these waterfowls prefer to sleep on the water and only sleep on land when they feel safe from predators.

Resting while flying is made easier for them by sleeping with an eye open, with the leader remaining fully awake at the point of the V while the other group members enter a state of unihemispheric slow-wave sleep. 

10. Oystercatchers

American oystercatcher
American oystercatcher | image by Lip Kee via Flickr | CC BY-SA 2.0

Scientific Name: Haematopus palliatus

When the tide is high, oystercatchers, which are found on mudflats, sandy beaches, and occasionally rocky shorelines, congregate on the ground to sleep in big groups.

As they frequently see humans and dogs in their environment, it has been observed that they must sleep with their eyes open to remain vigilant against potential threats. They become less prone to this “peeking” behavior as they grow more comfortable in their environment. 

11. Peregrine Falcons

Peregrine falcon perching
Peregrine falcon perching | image by David Merrett via Flickr | CC BY 2.0

Scientific Name: Falco peregrinus

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The peregrine falcon is a species of bird that’s well-known for its ability to fly at high speeds, with most individuals reaching speeds of over 180 miles per hour and some even reaching speeds of up to 200 miles per hour.

Although they’re known as birds of prey in their environment, they still need to stay alert for larger predators that might approach them. This is where being able to sleep with one eye not closed would be extremely beneficial to their survival. 


  • “Too hot to sleep? Sleep behavior and surface body temperature of Wahlberg’s Epauletted Fruit Bat”, C.T. Downs, A. Awuah, M. Jordaan, L. Magagula, T. Mkhize, C. Paine, E. Raymond-Bourret, L. A. Hart, PLoS One, March 16, 2015, pubmed.ncbi.nlm.nih.gov