The reason why many living things sleep and the evolutionary origins of sleep are among the great mysteries of neuroscience . All vertebrates and complex animals in general sleep . Humans, whales, mice, fish and fruit flies sleep, despite the fact that during that time they are left at the mercy of predators and unable to eat and reproduce , which is inexplicable to scientists. Why are they exposed in this way for much of the time? The theories handled suggest that sleep helps the brain to regenerate memory and learning and contributes to cellular recovery . But now it has been proven for the first time that even an organism without a brain or neurons spends time each day sleeping . This could indicate that the dream is evolutionarily older than previously thought.
And it is that until now science believed that having a brain was a necessary condition to disconnect and indulge in sleep . But it turns out that it is not necessary . A team from the California Institute of Technology (Caltech) has discovered that inverted Cassiopea jellyfish, creatures without a central nervous system, sleep just like we do . Jellyfish are cnidarians, fairly simple animals that live in aquatic environments, almost always marine. They can remain loose or in colonies, fixed to the substrate or free.
But until now there was no evidence that they needed to sleep. “This is the first case of sleep in animals without brains, ” says Paul Sternberg, a researcher at Caltech’s Howard Hughes Medical Institute (HHMI) and one of the study’s authors. Their findings, published in the journal Current Biology , advance the origin of sleep in the evolutionary tree of life, before the appearance of a centralized nervous system.
In Sternberg’s opinion, the big question is no longer why do we sleep but: ” Do all animals sleep? ” It is a controversial issue. So far, all vertebrates that have been studied do so , but there is no certainty that this will apply to all invertebrates. Fruit flies are known to dock, as is the Caenorhabditis elegans worm. But in more primitive animals like sponges and jellyfish it had not been tested until now.
Sternberg and his team set out to study the behavior of Cassiopea, a jellyfish that lives in areas of mud, mangroves and other warm, shallow waters , and has extended its habitat to the Mediterranean. Cassiopea does not look like the most common jellyfish . It is the size of a coin with black spots and lives upside down at the bottom of the sea, with its tentacles curled over its bell-shaped body.
But like other jellyfish, Cassiopeas contract and relax in a constant rhythm , so the researchers wondered if they ever interrupted this cadence. To verify this, they recorded with an iPhone the cnidarians put in tanks of salt water in a laboratory. After recording them for a minute at night and during the day, they counted the pulses and saw that at night the rate of contractions slowed, a sign that they could be sleeping .
They then created an image processing program to count the contractions of 23 jellyfish for six consecutive days and nights. The pulsating activity diminished at night, as they had previously observed, but if they put some food in the tanks the jellyfish would wake up quickly.
” It’s like the effect of the smell of coffee that invades our consciousness in the morning, ” says Sternberg. Such an easy awakening indicates that Cassiopea’s slow nocturnal behavior is due to sleep, not paralysis or coma. The researchers discovered a second sleep signal by forcing the sleeping jellyfish to move: they placed them inside a PVC pipe with a mesh bottom and then dipped the pipe into the tank, so the animals were resting on the mesh and not above the tank floor. They would then push the tube downward, forcing the jellyfish to lift the mesh and float in the water. They did not like the experience at all.
At night it was difficult for them to start swimming three times as long as during the day and they seemed somewhat dazed . This delayed response to stimulation is typical of sleeping animals. And like all of them, if you force the jellyfish to stay up at night, they pay it the next morning . Sternberg and his colleagues checked this by squirting small jets of water at them for six to twelve hours at night and then counting their body’s contractions the next day. The result was that they were less active after missing hours of sleep, and once they got it back and caught up, they returned to their normal rhythm. “The results suggest that Cassiopea sleeps, but for Sternberg this study raises new questions: Does it Neurons are needed to sleep? More than one cell? The researcher now intends to study sponges or even single-celled protozoa in search of the origin of sleep.
