How Do Dolphins Sleep Without Drowning?


As aquatic mammals, dolphins do not have gills for breathing under water. Like their fellow cetaceans whales and porpoises, they have to breach the water to breath oxygen in their lungs. For this reason, they need to live in the upper part of the world's oceans, running out of oxygen if they swim too deep for too long. While this may be easy enough when conscious, it raises the question how do dolphins sleep without drowning?
Dolphins have incredible evolutionary adaptations which allow them to sleep in the aquatic environment. As AnimalWised reveals, dolphin sleep patterns and behavior are quite different to terrestrial mammals. We look at how they manage to rest while swimming, as well as other important information about the dolphin sleep cycle.
Do dolphins sleep?
While sleep patterns and cycles differ greatly according to species, all animals will need to rest to some degree. The dolphin is no exception and they need to sleep for various reasons. The manner in which they sleep is adapted to their particular needs.
Terrestrial mammals tend to sleep for long continuous periods, although there is great variation among them. Unlike land mammals, aquatic mammals like the dolphin tend to sleep in a very fragmented way. They never enter REM sleep, the phase of deep sleep common in many land mammals. Instead, dolphins are able to rest thanks to a phenomenon known as unihemispheric sleep.
What is unihemispheric sleep?
Unihemispheric sleep means that one hemisphere of the dolphin's brain is active and conscious while the other is at rest. This ability allows dolphins to rest while still performing essential activities, such as swimming, breathing and being alert to dangers. For example, they can continue swimming in circles or near the surface to breathe, something which helps them avoid the risk of drowning while resting.
During sleep, the active hemisphere of the dolphin's brain maintains certain motor controls, allowing the dolphin to continue swimming and performing basic movements. This partial rest is not only a strategy to avoid predators, but also a way of adapting to the marine environment. It is necessary to keep moving while in water for the animal to survive.
For animals to sleep while in water, they need to have adaptations for breathing, something which hemispheric sleep is able to allow in dolphins. Learn more with our article asking how do dolphins breathe?
How do dolphins sleep without drowning?
As we have explained, dolphins have a unique adaptation when it comes to sleeping in the aquatic environment. Using unihemispheric brain activity, they can alternate which side of their brain is at rest. While one hemisphere is resting, the other remains active. This allows the dolphin to perform essential functions, such as breathing and navigation.
This sleep pattern is vital to their survival, as dolphins must be constantly alert and on the move to avoid dangers such as predators. They can maintain swimming activity, so they do not drown.
In terms of behavior, a sleeping dolphin is easily identifiable because it usually remains floating in the water or swims slowly in circles. In addition, dolphins can vary the intensity of their rest. While one half of the brain rests deeply, the other half maintains minimal cognitive function. This is sufficient to detect changes in the environment and react to threats, giving them the ability to be on alert without drowning.

Do dolphins sleep with one eye open?
Yes, dolphins can indeed sleep with one eye open. The eye opposite the active brain hemisphere will remain open, while the other eye will be closed or appear to be more relaxed. Coupled with the fact that a dolphin never completely stops moving, this ability can make it difficult for those unfamiliar with its behavior to observe that it is actually sleep.
Discover the different types of dolphins with our related guide.
What is the sleep cycle of dolphins?
The dolphin sleep cycle is unique among mammals. Humans have phases of deep and light sleep, but dolphins do not experience complete deep sleep. Instead, they alternate resting their brain hemispheres in cycles of several hours a day.
Dolphins typically spend about a third of their day in this state of unihemispheric sleep, as we mentioned earlier. They can rest for short intervals that are spread throughout the day and night, rather than having one continuous period of sleep.
In addition, baby dolphins have different sleeping patterns. During the first month of life, both calves and mothers remain active almost all the time to avoid predators and ensure the well-being of the newborns.
Learn about how dolphins are able to have babies with our article on dolphin mating and reproduction.
How much do dolphins sleep?
The amount of time dolphins spend sleeping varies by species and their respective environment. On average, dolphins sleep between 8 and 12 hours a day. This amount of sleep does not occur continuously, but is instead broken up into smaller periods of time throughout the day and night. This fragmented sleep pattern is due to a dolphin's need to keep moving in order to breathe, avoid predators and perform other vital functions such as feeding.
For example, some studies have found that dolphins in the wild sleep in short cycles of 15 to 20 minutes at a time, alternating between active brain hemispheres. In controlled environments such as aquariums, others can rest for longer periods of a continuous duration, although this is still less than what is needed for most other mammals.
Dolphin sleep behavior is also influenced by factors such as:
- Environmental security
- Presence of other members of the group
If a dolphin is in a safer area or in a group, it may be able to rest for longer. When they are in danger or in more hostile environments, their sleep time may be shorter and fragmented.
You can learn more about diet and other behaviors with our article sharing different characteristics of dolphins.

Where do dolphins sleep?
Dolphins sleep in water, but they do so in a way that always maintains the ability to move and breathe. Where exactly a dolphin sleeps will depend on the species and their natural habitat. Most dolphins are oceanic and live in saltwater environments, but certain dolphins such as the Amazon river dolphin live in freshwater bodies of water. Regardless of the type of habitat, they tend to sleep in areas where they can minimize threat.
With this in mind, we can say that dolphins sleep in the following conditions:
- Open ocean: some dolphins rest by swimming in circles in the open ocean, usually close to the surface. They form groups known as pods in which they take turns to rest and keep watch for possible threats.
- Coastal areas or lagoons: dolphins can also rest near the coast, in areas where the water is calmer and the risk of predators is lower. In these places, dolphins rest in more compact groups, something which offers them greater security while they sleep.
- In groups: dolphins are social animals and usually rest in groups. This grouping allows them to alternate resting while other members of the group are awake, providing mutual protection and security.
During their rest, dolphins may stay near the surface for easy access to air. They may also swim slowly near the bottom, depending on water conditions. Their ability to stay in motion during sleep also allows them to be alert to any changes in the environment that require their attention.
You can learn more about how dolphins are able to survive in often inhospitable environments with our article which asks how long do dolphins live?

If you want to read similar articles to How Do Dolphins Sleep Without Drowning?, we recommend you visit our Facts about the animal kingdom category.
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- Sekiguchi, Y., Arai, K., & Kohshima, S. (2006). Sleep in continuously active dolphins. Nature, 441(7096), E9–E10. https://doi.org/10.1038/nature04972
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