“Resting Your Brain” Is a Myth!? The Shocking Truth That Even Brainless Jellyfish Sleep Soundly… Experts Warn: “Sleeping Only 3 Hours Is Self-Harm”

The “Crucial Difference” Between Rest and Sleep

“Until now, sleep was thought to be a way to rest the brain, but recent research has shown that even brainless creatures like jellyfish and hydras sleep.”

This startling fact comes from Kazuhiko Kume, a professor at Nagoya City University who studies sleep. While sleep has traditionally been studied by measuring brain waves, he explains that it has recently come to be defined based on “behavior,” leading to an expansion of sleep research across a wide range of organisms.

As for what behaviors are classified as “sleep,” the specific criteria are the following three:

1. The presence of a characteristic posture or location  

For example, this includes bats hanging from ceilings or ducks tucking their beaks into their wings and standing on one leg atop a stump. Additionally, they have specific locations for sleeping, such as safe nests or warm spots.

2_ Reduced responsiveness

  If an animal is merely resting, it will react to minor stimuli or respond when spoken to. However, during sleep, it does not wake up easily.

3. The amount of sleep needed is generally fixed  

If sleep time is reduced, they may fall asleep during periods when they normally would not. Furthermore, their sleeping hours are generally fixed.

For example, the Sakasakurage jellyfish sinks to the seafloor at night, and the number of times it opens and closes its bell decreases to about two-thirds of the daytime rate.Furthermore, when they were continuously stimulated during the night, their daytime activity levels dropped significantly—possibly due to sleep deprivation. Based on these observations, it is believed that jellyfish are not merely “resting” but are actually “asleep.”

The Organisms That Mastered Sleep Survived

“Until now, sleep researchers believed that only organisms with brains slept. However, we’ve discovered that this is not the case.”

According to Professor Kume, the state of “sleep” is closely related to the day-night cycle.

“Organisms evolved gradually starting with cyanobacteria—primitive organisms similar to E. coli that appeared among the very first life forms on Earth. The reason cyanobacteria proliferated so extensively is that they were able to make effective use of both day and night.”  

Cyanobacteria used daylight to perform photosynthesis, which served as their energy source. Then, during the lightless night, they prepared themselves so they could be ready to function as soon as day broke. I believe that organisms capable of utilizing the difference between day and night—and using ‘sleep’ for survival—were the ones that prevailed.”

Then, as organisms developed brains during the course of evolution, they began to use sleep time to “organize information.”

“We’ve come to understand that even small brains, like those of fruit flies, process information while sleeping. In humans, when we don’t get enough sleep, we can’t process information properly, and our brains become confused.”

On the other hand, it is believed that jellyfish, which lack brains, likely sleep to rest their bodies and conserve energy.

Sleeping “three hours” is nothing short of self-harm

The amount of sleep needed and how it is obtained vary from species to species.

Whales and dolphins use “hemispheric sleep,” in which they rest one half of their cerebrum at a time, allowing them to sleep while swimming. Migratory birds are also able to continue flying through the air by utilizing this “hemispheric sleep.”

“But even though they alternate between the left and right sides, it’s only for 10 or 20 seconds at a time, and it seems they sleep for only one to two hours while migrating. However, once they finish migrating and build their nests, they sleep for about nine hours. I think that’s how they’ve evolved.”

In humans, the amount of sleep needed varies by age. Children aged 1–2 require 11–14 hours; around age 15, about 8 hours; and for people in their 30s and 40s, around 7 hours is considered appropriate.

“However, few people in their 30s and 40s are likely getting enough sleep,”

According to Professor Kume, you can tell whether you’re getting enough sleep by how long you sleep on your days off.

“If you sleep more than two hours longer than usual on your days off, you’re definitely sleep-deprived. That said, as long as you can maintain that pattern without straining yourself, you’ll be fine.”

The real danger arises when people who are chronically sleep-deprived on weekdays also find themselves unable to sleep on their days off.  

“When you’re sleep-deprived, your parasympathetic nervous system becomes overactive. Since your body has to counteract this, your sympathetic nervous system also becomes overactive. If chronic sleep deprivation continues, this state of constant heightened activity persists, leading to high blood pressure. If this condition lasts for more than three months, the risk of developing depression increases.”

Prime Minister Takaichi has publicly stated that she “sleeps only three hours,” but this is nothing short of self-harm. It could even lead to confusion in the brain and result in poor judgment. I sincerely hope she gets enough sleep first and foremost.

▼ Kazuhiko Kume: Professor at the Graduate School of Pharmaceutical Sciences , Nagoya City University. Molecular biologist. Physician. He has served as a visiting researcher at Harvard University and Tufts University, and as an associate professor at the Institute of Developmental Medicine, Kumamoto University, before assuming his current position.His specialty is neuroscience. He researches the mechanisms of the biological clock and sleep. He also treats sleep disorders in internal medicine and psychiatry. His publications include *The Molecular Biology of Time* (winner of the Kodansha Science Publishing Award) and *Jellyfish Sleep Even Though They Have No Brain* (Asahi Shimbun Publications).