Where Do Dolphins Sleep?
Since dolphins need to breathe, they sleep along the surface of the water or rise to the surface occasionally while resting along the bottom of a shallow area. Their sleeping pattern is very different from other animals except for whales, who have a similar respiratory system. Dolphins either sleep alone or in pairs.
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Identification
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Dolphins, like whales, do not have involuntary breathing respiratory systems like humans and other animals do. Thus, they must be conscious to breathe. They can stay underwater for about 30 minutes and then need to rise to the surface to breathe through the blowhole. Because of this, dolphins cannot go into a full sleep pattern, and instead only half of their brain sleeps at a time, as confirmed by electroencephalograms. They are in this state on and off for about eight hours a day. This semi-conscious state also keeps them alert enough to sense predators.
Types
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A common way that dolphins sleep is to swim very slowly along the surface, hardly moving, which allows them to continue breathing. Italian researchers studying dolphins in the Mediterranean have studied this phenomenon, and found that the sleeping dolphins usually swim in pairs side by side. They typically close one eye, and although this does not always coordinate with which particular side of the brain stays alert, it does coordinate with their partner's open eye. Usually the two open eyes are looking at each other. If the dolphins switch positions, they also change which eye each has open. This type of sleeping behavior is called "rest swimming" or "logging." It is common in wild dolphins, and usually associated with calm water.
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Function
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In another type of behavior, dolphins stop swimming and rest at the surface with the blowhole above the water. This is typical for lone dolphins, and again, one eye stays open. This eye stays open for an average of an hour, and then it closes and the other opens. Sleeping dolphins also rest on the bottom in shallow water and surface now and then.
Considerations
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In all three of these sleep patterns, the animals show no interest in their environment and do not make any sounds. They actually appear to be sound asleep, except for the open eye and the minimal swimming movement.
Features
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The only sleep behavior seen in juvenile dolphins is rest swimming with the mother, staying in physical contact most of the time. Calves nurse for about 18 months, but a mother and offspring may continue swimming together for several years. There is no record to date of a calf resting at the surface.
Theories/Speculation
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Rapid eye movement sleep, the type of sleep in which dreams occur, has been recorded very rarely in dolphins. Some scientists speculate that dolphins do not dream at all, while others theorize that dolphins dream for very tiny amounts of time, probably in seconds. REM is also associated with atonia, a type of paralysis that stops animals from acting out their dreams. It would be very difficult for dolphins to dream at length since they are usually swimming to some degree even while sleeping, and cannot do so if experiencing atonia.
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Comments
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Kiri asked me if dolphins sleep and now we know the answer. They do, with half a brain, and usually with one eye open and in pairs, and they probably don't dream at all or if they do, it's just a tiny dream.
