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How Do Dolphins Sleep?

A Scientific Look at the Dolphin's Sleep Habits

How Do Dolphin's Sleep? A Scientific Look at the Dolphin's Sleep Habits

Dolphins are members of the marine mammal group called cetaceans, which also include whales, orcas, and porpoises. As water dwellers and oxygen breathers, these mammals have to sleep in a way that prevents them from drowning. Dolphins sleep in a way that is different from how you and other land mammals sleep. When you sleep, your bodily functions include the inactivation of all voluntary muscles, but you are also equipped with a respiring reflex that allows you to breathe automatically to prevent suffocation during sleep. Dolphins aren’t equipped with that respiring reflex, so their way of sleeping is very different and important to their survival.

Unihemispheric Slow-Wave Sleep

Unihemispheric slow-wave sleep, or USWS, is a sleep phenomenon that exists when one hemisphere of the brain is asleep, and the other hemisphere remains conscious, which is in contrast to how you sleep, with both hemispheres being asleep at once. During sleep, dolphins will rest one hemisphere of their brain and close the opposite eye, so when they rest the right side of their brain, the left eye will stay open, and vice versa. Dolphins alternate resting hemispheres during their sleep periods to ensure both hemispheres receive the same amount of sleep. When this occurs, the hemisphere that is awake monitors its environment, controls breathing functions, and filters waking knowledge to the sleeping side. USWS is a deep sleep that is slow-wave that is present in the resting hemisphere of the brain, which is believed to help the brain unify new and old memories and revive itself from daily living.

U.S. Navy scientists of the Marine Mammal Program observed dolphins for five days to determine if sleep deprivation affected dolphins due to USWS, and it was found the dolphins were as receptive and attentive as before observation began with no evident consequences to their health or mentality. In separate research done by National Marine Mammal Foundation, it has also been found that dolphins are capable of staying awake for at least two weeks, also without noticeable effects.

Dolphin Sleep Patterns Observed

Dolphins have been observed in captivity while in zoos and aquariums, as well as studied in the wild, and it’s been determined that they have two basic ways of sleeping, either resting still or rest swimming. Dolphins in captivity have been observed to sleep at the bottom of their pools or tanks, periodically surfacing for air. While in the wild, they may rest by hanging vertically in the water or lying on their side on the surface of the water, which is called logging. They may also sleep while slowly swimming next to another animal, which is known as rest swimming. According to a 2008 article in the journal, Neuroscience & Behavioral Reviews, it has been documented that during a 24 hour period, each hemisphere of the dolphin’s brain will get approximately four hours of sleep. There is also evidence that dolphins undergo light rapid eye movement or REM sleep cycles, which suggests a possibility that dolphins may be able to dream as you do during REM sleep. This finding contradicts unihemispheric slow-wave sleep, also known as asymmetric slow-wave sleep, when one-half of the brain is in deep sleep, which is a form of non-rapid eye movement sleep.

Evolved Sleeping Method For Dolphins

An indication that USWS is an evolutionary incidence in the brain is the release of the neurotransmitter acetylcholine, which happens when there is a need for it, especially when it contributes to the life of the dolphin. There are three main reasons that dolphins have needed to adapt to sleep in this manner. First, the dolphin consciously decides when to breathe and if one hemisphere wasn’t awake to control breathing, it would drown. The second reason, unihemispheric slow-wave sleep, is a defense mechanism which is a provision to the dolphin’s endurance by permitting them to stay alerted to predators, obstacles, and other animals while they rest. Third, this type of sleep enables the dolphin to maintain certain physiological activities, such as muscle function, which is imperative for the warm-blooded dolphin to retain the body heat it needs to survive in freezing waters.



“Dolphins sleep with half their brain, can stay awake for at least 2 weeks.” EarthSky. N.p., n.d. Web. 15 Mar. 2017.
“How do dolphins sleep?” WDC, Whale and Dolphin Conservation. N.p., n.d. Web. 15 Mar. 2017.
“Unihemispheric slow-wave sleep.” Wikipedia. Wikimedia Foundation, 16 Feb. 2017. Web. 15 Mar. 2017.

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