Evolution of sleep before brains. Hydras are living proof

hydra simple creature. Its tubular body is less than half an inch long, with a foot at one end and a mouth at the other. The foot clings to an underwater surface – perhaps a plant or rock – and the mouth, enclosed in tentacles, is entrapped by water fleas. It doesn’t have a brain, or even much of a nervous system.

and yet, New search appears, Sleeps. Studies by a team in South Korea and Japan have shown that the hydra periodically falls into a resting state that meets the basic criteria for sleep.

On the face of it, this may seem unlikely. For more than a century, researchers who study sleep have searched for its purpose and structure in the brain. They explored sleep links with memory and learning. They number the neural circuits that drive us down into an oblivious slumber and get us out of it. They recorded the apparent changes in the brain waves that mark our passage through different stages of sleep and tried to understand what drives them. Mountains of research and the daily experience of people bear witness to human sleep brain connection الاتصال.

But an opposite view has emerged to this brain-centric view of sleep. Researchers have noticed that the molecules produced by muscles And the some other tissues Outside the nervous system can regulate sleep. Sleep affects the metabolism extensively in the body, indicating that its effect is not only neurological. And the body of work that has been quietly but constantly growing for decades has shown that simple organisms with less brains spend a significant amount of time doing something very similar to sleeping. Sometimes their behavior is only categorized as ‘sleep-like’, but as more details are revealed, it becomes less and less clear why this distinction is necessary.

It appears that even simple creatures – including, now, the brainless hydra – can sleep. The interesting implication of this finding is that the original role of sleep, buried billions of years ago in the history of life, may have been very different from the standard human conception of it. If sleep does not require a brain, it may be a much broader phenomenon than we expected.

sleep recognition

Sleep is not the same as hibernation, coma, drunkenness, or any other state of lethargy, as the French sleep scientist Henri Piéron wrote in 1913. Although everything implies an outwardly similar absence of movement, each has distinct qualities, and this discontinuity Our daily conscious experience has been particularly ambiguous. Going without it made one blurry, confused, incapable of clear thinking. For researchers who wanted to learn more about sleep, it seemed necessary to understand what it does to the brain.

And so, in the middle of the 20th century, if you wanted to study sleep, you became an expert reader of electroencephalograms, or EEGs. Placing electrodes on humans, cats or mice allowed the researchers to say with clear accuracy whether a person was asleep and what stage of sleep they were in. This approach produced many insights, but left a bias in the science: Almost everything we learned about sleep came from animals that could be fitted with electrodes, and sleep was increasingly characterized in terms of the brain activity associated with it.

this is frustrating Erin Tobler, a sleep physiologist working at the University of Zurich in the late 1970s who began studying cockroach behavior and was curious to see if invertebrates like insects sleep like mammals. After reading Piéron and others, Tobler realized that sleep can be defined behaviorally as well.

She distilled a set of behavioral criteria to define sleep without EEG. The sleeping animal does not move. It is much more difficult to wake up than simply to wake up. He may take a different position than when he wakes up, or he may be looking for a specific place to sleep. Once awake, he behaves normally and is not sluggish. Tobler added a criterion of her own, drawn from her work with mice: A disturbed sleeping animal would subsequently sleep longer or deeper than usual, a phenomenon called sleep homeostasis.

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