Biologists first discovered the rhythms of sleep in fish, characteristic of humans. This suggests that the sleep phases arose much earlier than previously thought — perhaps 450 million years ago, according to a recent study published in the journal Nature.
In humans and many other living beings, the phases of deep sleep alternate with the phases of so-called REM sleep, when the sleeping person's eyes move rapidly under the eyelids. At this time, unlike the deep sleep phase, the brain is almost as active as awake, and people who are awakened at this stage almost always report that they have just had a dream.
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An international research group led by Philip Morren of Stanford University (USA) was able to detect such sleep rhythms in fish for the first time, in particular in zebrafish aquarium fish, which were only two weeks old. Since the ancestors of these fish lived long before the earliest mammals appeared on Earth, scientists assume that the separation of sleep into two phases occurred much earlier than previously thought — perhaps 450 million years ago.
What exactly happens at different stages of sleep and why sleep is so important for all living beings is not yet precisely known. However, there are several theories. Watching the fish, scientists found that lack of sleep leads to damage to nerve cells, and malfunctions accumulated in the genome, while zebrafish did not sleep. If they slept, the genetic material in the nerve cells was restored.
When biologists deprived the fish of sleep, the damage accumulated sharply. And if they artificially damaged DNA with chemicals, the fish subsided until it was restored. Thus, an important function of sleep may be the cessation of activity in the neurons, so that the potential “powers” of the cells for recovery are released.
Even externally, the dream of danios and people is so similar that non-specialists can recognize a dormant fish: it still stiffens in the water and limply lowers the tail fin. It turned out that they are difficult to wake up. Like a deeply sleeping man, danios do not immediately wake up.
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It seems that the fish can be a convenient object to study sleep and even search for drugs from its disorders.
Still, there are differences between the sleep of people and animals. The most obvious is the duration of sleep. Most people sleep between six and eight hours a day. A chimpanzee, the closest relative of a man to be in shape, sleeps for almost twelve hours. Why these differences exist is not yet known. However, scientists have found that 25% of people's sleep time is a phase of REM sleep. In chimpanzees and other primates – only about 5%. Apparently, scientists suggest, in the course of evolution, people could optimize their sleep thanks to REM phases.
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