First of all, dolphins do sleep, but it’s not the type of sleep land mammals have.

Dolphins are conscious breathers, which means they have to think about when to go up to the surface and take a breath. Since they don’t breathe autonomously and have to plan every single breath, they undergo a form of sleep called “unihemispheric slow-wave sleep.” Also known as deep sleep, slow-wave sleep is a type of sleep thought to help the brain consolidate new memories and recover from its daily activities. In electroencephalographic experiments, sleeping bottlenose dolphins have been reported to show signs of wakefulness (low voltage, fast activity waveforms) in one hemisphere and sleep (high voltage, slow wave) in the opposite hemisphere.

When resting, a dolphin will shut down only one hemisphere of its brain and close the opposite eye (the left eye will be closed when the right half of the brain sleeps, and vice versa). The active side of the brain controls movement, breathing, pays attention on the environment and watches our for dangers.

Dolphins don’t have a typical circadian rhythm as land animals do, they choose to stay active or rest in adjustment to their pray’s activity. Within a 24-hour period, each half of the brain gets about 4 hours of slow-wave sleep.

While asleep dolphins can’t use echolocation, since the whole brain is needed for that. In order to not be in danger while sleeping, dolphins swim in packs and individuals interchange, the ones that aren’t resting, keep a lookout for danger. They never rest all at once. In case of noticing danger while asleep, dolphins immediately wake up completely and react accordingly. A very important factor that disturbes their rest is noise.

Sleep behaviour

Sometimes, dolphins will hang motionless at water surface during sleep, while other times, they may swim slowly. Some species, such as pilot whales, exhibit a behaviour called logging, when resting. Meaning they stay on the surface and don’t perform any movement themselves, they just move with the water flow. Other species take shallow dives and cruise along beneath the water surface in between breaths. Spinner dolphins in the Red sea of Egypt have to feed at night because that’s the time their prey comes within their diving range. Being active at night, they have to rest during the day. To avoid their predators – sharks, they rest in shallow lagoons or reefs with a light sandy bottom.

There are three main reasons why dolphins have evolved this sleeping style. First, dolphins have to keep half of their brain active, because their breathing is always consciously controlled. Second, unihemispheric slow-wave sleep allows the animals to look out for danger while they rest. Third, this type of sleep allows the dolphin to keep up certain physiological processes, such as muscle movement, that helps the warm-blooded mammal maintain the body heat it needs to survive in the frigid ocean.

Breathing

Other methods help marine mammals to hold their breath longer than other types of mammals can. Marine mammals can take in more air with each breath, as their lungs are proportionately larger than those in land mammals. In addition, they exchange more air with each inhalation and exhalation. Their red blood cells also carry more oxygen. When diving, marine mammals’ blood travels only to the parts of the body that need oxygen – the heart, the brain and the swimming muscles. Digestion and any other processes have to wait.

Finally, these animals have a higher tolerance for carbon dioxide (CO2). Their brains do not trigger a breathing response until the levels of CO2 are much higher than what, for example, humans can tolerate. These mechanisms, part of the marine mammal diving response, are adaptations to living in an aquatic environment and help during the process of sleeping. Cetaceans reduce the number of breaths they take during rest periods; a dolphin might average 8 to 12 breaths a minute when fairly active only to have their breathing rate drop to 3 to 7 per minute while resting.

 

References

Encyclopedia of marine mammals

https://www.livescience.com/44822-how-do-dolphins-sleep.html

https://www.scientificamerican.com/article/how-do-whales-and-dolphin/

 

 

Disclosure: This post contains affiliate links, which means I receive a commission if you purchase a product through these links. Don’t worry – the price is still the same for you. Visit my disclosure page for more information.