Herbie Murray
Dolphins have to be conscious breathers, meaning they must choose to come to the surface to breathe. If they don't, they could drown. So, how do they sleep? Dolphins have two hemispheres in their brain, and they shut down only half of their brain when they sleep, along with the opposite eye. The other half of the brain stays awake and alert to watch for predators and obstacles and to signal when to rise to the surface for a breath of air. After about two hours, they will switch, resting the side of the brain that was previously awake. This type of sleep is called unihemispheric slow-wave sleep.
| Characteristics | Values |
|---|---|
| Sleep pattern | Unihemispheric slow-wave sleep |
| Brain activity | One hemisphere of the brain sleeps at a time |
| Eye activity | The eye opposite to the sleeping hemisphere remains closed |
| Alertness | The other hemisphere stays awake at a low level of alertness |
| Functions of the alert hemisphere | Monitors the environment, looks out for predators, signals when to rise to the surface for a breath of air |
| Switching hemispheres | Every two hours |
| Average sleep per day | A third of their day |
| Breathing rate when awake | 8-12 breaths per minute |
| Breathing rate when asleep | 3-7 breaths per minute |
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What You'll Learn
Dolphins are conscious breathers
To get the rest they need, dolphins have a unique ability to sleep with only one half of their brain at a time. This is known as unihemispheric sleep. While one hemisphere sleeps, the other stays alert to remind the dolphin to breathe and watch out for danger. Dolphins also close one eye when they sleep—the left eye when the right half of the brain is sleeping and vice versa.
Dolphins typically spend about two hours at a time with one side of the brain at rest before switching to the other side. Over a 24-hour period, each hemisphere gets around four hours of slow-wave sleep. This sleep pattern allows dolphins to keep breathing and stay safe from predators while still getting the necessary rest.
In addition to their unihemispheric sleep, dolphins have other adaptations that help them rest effectively. As highly social animals, they often pair up and swim side-by-side while resting, providing safety in numbers. They also slow their breathing rate when sleeping, becoming more efficient in their energy recovery. Furthermore, dolphins have efficient immune systems, brain plasticity, the ability to thermoregulate, and a higher tolerance for carbon dioxide, all of which compensate for their lack of complete sleep.
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They enter a state of 'unihemispheric slow-wave sleep'
Dolphins have a unique way of sleeping, called unihemispheric slow-wave sleep. This means that they enter a state of sleep where only one hemisphere of their brain sleeps at a time, while the other half remains awake and alert. Unihemispheric slow-wave sleep is also called unihemispheric sleep, and it is believed that dolphins periodically switch the active side of their brain every two hours, ensuring each side gets adequate rest.
During this process, the sleeping hemisphere of the brain shuts down, along with the opposite eye. The awake side remains vigilant, monitoring the environment for potential dangers and obstacles, and crucially, signalling when to rise to the water's surface to breathe. This is especially important because dolphins are conscious breathers, meaning they must consciously decide to breathe, and if they were to enter a deep sleep like humans, they would risk suffocating or drowning.
Unihemispheric slow-wave sleep allows dolphins to remain alert and safe while resting. They can quickly shift from this state of sleep to full wakefulness if a threat is detected. This method of sleeping is not unique to dolphins, and many other animals, including birds, exhibit similar sleeping patterns.
Dolphins typically spend about two hours with one side of their brain at rest before switching, and they generally sleep at night, remaining active late into the night. Observations of dolphins in aquariums, zoos, and the wild have revealed that they spend an average of about a third of their day sleeping in this manner.
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They rest horizontally or vertically
Dolphins have two hemispheres in their brain, just like humans. However, they cannot afford to rest their entire brain at once, as they need to remain conscious to breathe and be vigilant of their surroundings. Therefore, dolphins have evolved to rest one half of their brain at a time, a process known as unihemispheric sleep. During this time, the opposite eye will be closed. After a couple of hours, the dolphin will switch, resting the side of the brain that was previously active.
Dolphins can rest horizontally or vertically. When sleeping, they may lie motionless at the surface of the water, breathing regularly, or they may swim very slowly and steadily, close to the surface. In shallow water, dolphins have been observed to sleep on the seabed, rising to the surface at regular intervals to breathe. Adult male dolphins, which generally travel in pairs, will often swim slowly side-by-side as they sleep.
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They sleep while swimming slowly
Dolphins have a very different sleep pattern to humans. Unlike humans, they do not have prolonged periods of unconscious sleep and are always aware of their surroundings, even when they are sleeping. This is because they do not have a breathing reflex that takes over when they are unconscious, so they must remain conscious even during rest, or they risk suffocating or drowning.
To get around this, dolphins have a unique evolutionary adaptation that allows them to rest: they enter a state of sleep called 'unihemispheric slow-wave sleep' or USWS. This means that only one hemisphere of their brain (and the opposite eye) sleeps at a time, while the other half remains alert. The attentive side is used to watch for predators, obstacles and other dangers in the environment, and to signal when to rise to the surface for a breath of air.
Dolphins generally spend about two hours resting one side of their brain at a time, and they may take a few of these rest periods during the day or night. They often lie motionless at the surface of the water, breathing regularly, or they may swim very slowly and steadily, close to the surface.
In shallow water, dolphins sometimes sleep on the seabed, rising regularly to the surface to breathe. Adult male dolphins, which generally travel in pairs, often swim slowly side by side as they sleep. This is known as 'echelon swimming', and females and young dolphins may also rest in the same general area or pair up with a companion animal.
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They slow their breathing rate while sleeping
Dolphins have a unique way of sleeping, which is very different from humans. Unlike humans, dolphins do not have prolonged periods of unconscious sleep. Instead, they practice unihemispheric slow-wave sleep, where one hemisphere of their brain sleeps at a time while the other half remains alert. This allows them to continue breathing consciously and remain aware of their surroundings, including potential dangers in the environment.
To facilitate this type of sleep, dolphins slow their breathing rate while sleeping. Their breathing rate drops from 8 to 12 breaths per minute when they are awake to 3 to 7 breaths per minute during rest. This slower breathing rate helps them recover energy more efficiently while they sleep.
The unihemispheric slow-wave sleep pattern allows dolphins to quickly switch from a state of sleep to full wakefulness if needed, such as in the presence of a predator. They typically spend about two hours resting one side of their brain before switching to the other side. This ensures that each hemisphere of their brain receives adequate rest.
In addition to slowing their breathing rate, dolphins have other adaptations that aid their sleep. They are highly social animals and often pair up to swim side by side while resting. This provides safety in numbers, as one dolphin can remain more alert and watch for potential dangers while the other sleeps.
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Frequently asked questions
Yes, dolphins do need sleep. They enter a state of sleep called 'unihemispheric slow-wave sleep' or USWS, where one half of the brain remains alert to watch for predators, signal when to rise to the surface for a breath of air, and react to danger.
Dolphins have to consciously decide to breathe, so they must remain conscious even during rest. They enter a state of USWS, which allows them to rest without losing consciousness. They also slow their breathing rate down from 8-12 breaths per minute while awake to 3-7 per minute while sleeping.
Dolphins generally sleep at night, but only for a couple of hours at a time. They are often active late at night, possibly to feed on fish or squid. Based on electroencephalogram (EEG) readings, bottlenose dolphins spend an average of 33.4% of their day asleep.
