Sleep mechanisms may precede brain evolution, organisms may default to sleep

For many years, it has been thought that 'sleep is a system controlled by the brain, and animals without a brain do not sleep.' However, in recent years, there have been multiple reports of evidence that brainless organisms also sleep, such as the fact that 'brainless jellyfish sleep'. Science , an academic journal, summarizes research on such 'sleep of animals without a brain'.

'If it's alive, it sleeps.' Brainless creatures shed light on why we slumber | Science | AAAS
https://www.science.org/content/article/if-alive-sleeps-brainless-creatures-shed-light-why-we-slumber

Many studies on sleep have been conducted for a long time, and in the 1950s and 1960s, studies using 'polysomnography ' that combines brain waves, eye movements, electrocardiograms, electromyograms, etc. were actively conducted, and 'humans'. There are REM sleep and non-REM sleep in sleep. ' In addition, by observing animal behavior and physiological analysis, 'some marine mammals sleep while swimming' 'some seabirds sleep in flight by sleeping half of their brains' 'bats sleep 20 hours a day' It has become clear that.

However, the above research was conducted on animals with brains, and the question 'Do animals without brains such as hydras and jellyfish sleep?' Has existed for many years. In recent years, Hydra and jellyfish with a simple nervous system in order to resolve this question, sponge, flat animal sleep study of the like have been made more.

For example, a research team at the California Institute of Technology observed the day and night behavior of jellyfish and found that 'brainless jellyfish also sleep.' Biologist Paul Sternberg said, 'Jellyfish are creatures that we thought had a'primitive nervous system', and the results of this experiment are very important.'

Experiments show that brainless jellyfish also sleep-GIGAZINE

In addition, Assistant Professor Taichi Ito and Mr. Hiroyuki Kanaya of Kyushu University (enrolled in the Faculty of Science, Kyushu University at the time of research) conducted research on sleep for hydra, and 'sleep exists in hydra with a primitive nervous system' and 'in hydra'. We confirmed that the effects of sleeping pills are widespread, and that hydra expresses genes related to sleep. Based on the research results, the research team proposes that 'animals may have acquired sleep at the mechanism level before the evolution of the brain.'

Where is the origin of our sleep? | Research Results | Kyushu University (KYUSHU UNIVERSITY)
https://www.kyushu-u.ac.jp/ja/researches/view/504/

In addition, it has been observed that the behavioral speed of placozoa, which is said to be 'the simplest animal on the earth', slows down at night, and at Amphimedon queenslandica , a type of sponge that does not have a complicated nervous system, ' It has been confirmed that the expression of genes known to be involved in sleep switches in a 24-hour cycle, and there is a lot of evidence that 'animals without a brain also sleep'.

From these research results, the idea that 'sleep is a system that has evolved from the time before the emergence of vertebrates' is becoming widespread. 'Until we evolved the way we respond to the environment, early organisms didn't'react'. We evolved sleep,' said Paul Shaw, a neurobiology researcher at Washington University in St. Louis. I think it's an evolution of arousal, not 'I think,' he said, pointing out that the organism's default state was 'sleep.'