The ancestors of cetaceans, such as dolphins, were mammals that evolved on land. 55 million years ago, those animals returned to aquatic life, still as mammals, and developed a special respiratory strategy.
All terrestrial mammals have autonomous breathing, that is, it is not consciously controlled, unlike aquatic mammals, specifically cetaceans, which have a respiration conscious and decide when they need air. Even when they are resting on the surface, they only breathe about three times a minute, with very fast and deep breaths, filling the lungs to 80 or 90 percent of their capacity.
Because of their need to swim and have to perform this type of breathing, it is not surprising that many people have wondered how do dolphins manage to sleep in the waterIn this article on our site, we will discover how dolphins breathe when they sleep or how these animals sleep.
What is sleep?
To understand how dolphins sleep, we must first know how the sleep-wake process is in mammals. The waking state and the sleeping state can be easily distinguished at the physiological and behavioral levels.
The sleep process has two phases: slow-wave sleep or non-REMand rapid wave sleep or REM While awake, the encephalographic activity is desynchronized, it shows waves of low amplitude but high frequency, unlike this activity during the sleep process that is synchronized, the amplitude of the waves increases and their frequency decreases.
During the non-REM phase, the muscular activity of the body decreases gradually, until it is annulled in the REM phase, which produces muscular atony from the neck down(there are no responses from the muscles of the body). In addition, in the REM phase rapid eye movements occur and the body temperature drops.
So how do dolphins sleep?
Unihemispheric sleep
It was in the 70's when a group of researchers from the USSR discovered how dolphins sleepDespite the fact that there are two phases during sleep, only one of them is known to exist in dolphins, non-REM, and it occurs in a unihemispheric manner, this means that, when a dolphin sleeps,only "turns off" one of the hemispheres of the brain , while the other continues with its waking activity or, to put it another way, one hemisphere of the brain is desynchronized (awake) and the other synchronized (asleep).
Passing from an awake state to another asleep occurs gradually, that is, while one hemisphere goes to sleep the other one wakes up, so we can find intermediate states in which one hemisphere is half alert half asleep.
REM sleep phase has not been identified in dolphins but it has been in some cetaceans and, surprisingly, it is not shown in a unihemispheric way, but in the whole brain.
Dolphins sleep with one eye open
Unihemispheric sleep in dolphins seems to occur mainly at night, in the second half of the day and at dusk. Studies show that both hemispheres rest the same number of hours.
Having this type of dream brings about a series of behaviors that allow the dolphin to continue interacting with the outside world. For example, one of these behaviors is that dolphins sleep with one eye open When the right hemisphere of the brain enters non-REM, the left eye closes and, when the left hemisphere sleeps, the right eye closes.
While a dolphin sleeps, you can continue doing all the activities you want, rest on the surface of the water, breathe, swim or communicate.
Why don't dolphins have unihemispheric REM sleep?
We might think that dolphins do not have REM sleep because during this phase the body goes into muscle atony and the dolphin could sink, but scientists are not so clear. In fact, if there were unihemispheric REM sleep, only half of the body would become atonic and there could be compensatory mechanisms to continue interacting with the environment.
The current hypothesis that has the most followers is that a dolphin could not have unihemispheric REM sleep because it could not know the difference between dreams and realityHalf of your brain would be analyzing information from the real world and the other half, information from the dream world. If this happened they would be easy prey for predators.
