Sleeping Without a Brain? What the Upside-Down Jellyfish Teaches Us About the "True Purpose of Sleep"

Sleeping Without a Brain? What the Upside-Down Jellyfish Teaches Us About the "True Purpose of Sleep"

Sleep is mysterious. It leaves us defenseless, halts foraging and reproduction, and makes us vulnerable to predators. Yet, sleep persists across a wide range of animals. Why can't we let go of such a "dangerous behavior"? An unexpectedly intriguing creature offers a clue to this question.


The main character is the "upside-down jellyfish." This jellyfish is characterized by its unique behavior of floating with its bell facing downward while pulsating on the seabed. Research has observed that this upside-down jellyfish enters a "sleep-like state" at night, where its pulsation tempo slows, and its reactions become sluggish. Moreover, it exhibits behaviors akin to taking short naps during the day and "making up" for disturbances at night.


What is crucial here is that jellyfish do not possess a "brain" like ours. Their nervous system is more akin to a "nerve net" spread thinly across their bodies, reacting to the environment without a central command center. Yet, they seem to "rest when they should." Is sleep a byproduct of the brain's advanced functions (like dreaming and memory consolidation), or is it a more fundamental survival strategy? The axis of this debate shifts to a much deeper level.


What exactly must be fulfilled to define "sleep"?

Simply closing your eyes and staying still does not constitute sleep. Whether an animal is asleep is generally determined by several criteria, with "decreased responsiveness" being a representative one. This means that the animal reacts slower or less readily to external stimuli than usual.


The research team filmed the behavior under infrared light and measured reaction speeds by exposing the jellyfish to white light or food stimuli. The results showed that when the upside-down jellyfish maintained a slow pulsation for a certain period, its reactions became sluggish. A similar tendency was observed in a type of sea anemone (the starlet sea anemone), where reactions slowed when it remained almost motionless for a certain period. This indicates that the "sluggishness during sleep" is also present in cnidarians.


Moreover, sleep exhibits "homeostasis." After sleep deprivation, there is an increase in sleepiness, and longer sleep compensates for the deficit—phenomena akin to "catching up on sleep" or "rebound sleep." The research observed that intentionally disrupting rest with water currents led to an increase in subsequent sleep time. The logic of "feeling sleepy the day after sleep deprivation" in humans also appears in animals with only a nerve net.


Core: Does being awake damage neuronal DNA, and does sleep promote its repair?

The central topic here is the hypothesis that "sleep = time for neuronal DNA maintenance."


The key points are as follows.

  • During wakefulness and activity, DNA damage (such as breaks) increases in neurons.

  • After rest or a sleep-like state, indicators of this damage decrease.

  • Sleepiness (sleep pressure) itself may be heightened by DNA damage or cellular stress.


The research team visualized traces of DNA damage in neurons using specific staining methods and tracked changes according to time and state. In the upside-down jellyfish, damage increased during continued activity and tended to decrease after extended rest. A similar trend was reported in sea anemones, suggesting a narrative where "the more awake, the more damage accumulates → rest leads to recovery" across different species.


Furthermore, to explore causality, experiments were conducted using ultraviolet (UV-B) rays, which easily damage DNA. Damage increased in a short time, followed by increased rest. After resting, the damage approached baseline levels—results that support a cyclical model where "damage induces sleep, and sleep aids repair."


How ancient is the "origin of sleep"?

Cnidarians (jellyfish, sea anemones, corals, etc.) are located quite close to the root of the animal phylogenetic tree. They are believed to have diverged from bilaterians (including insects and vertebrates, like us) approximately 600 to 700 million years ago. If the core function of sleep was already established at the cnidarian stage, it is likely that sleep was not a "convenient function born after the brain developed," but rather a "basic function necessary from the point when the nervous system was formed."


An interesting aspect here is the discussion of the "purpose" of sleep. Sleep has been proposed to offer multifaceted benefits, such as energy conservation, memory consolidation, and immune regulation. The current findings do not negate these but rather suggest a prioritization narrative: "The first necessity might have been the maintenance of neurons." Neurons are generally difficult to proliferate and challenging to replace once damaged. Therefore, the hypothesis that a regular "repair time" was necessary is evolutionarily plausible.


Does melatonin work on "brainless animals" too? A small shake to common sense

Another element that stimulates discussion is melatonin, often associated with sleep and circadian rhythms. We have the image that "melatonin increases when it gets dark, making us sleepy." However, experiments showed that adding melatonin to the water increased rest even in upside-down jellyfish and sea anemones during their "natural activity times."


While there is a belief that "the sleep-inducing effect of melatonin evolved in vertebrates with complex brains and circadian clocks," there is also a possibility that it functioned as a "rest switch" from a much earlier stage. Not only is the history of sleep ancient, but the chemical mechanisms that regulate sleep might also be older than imagined.


Reactions on social media: Surprise and agreement with "sleeping without a brain," and jokes

This topic is easily spread on social media. The reason is simple: the phrase "sleeping without a brain" is powerful. The reactions can be broadly categorized into three noticeable types.


1) Simple question: Why can't repairs be done while awake?

On overseas forums, the straightforward question "Why can't repairs be done while awake?" ranks high. Based on the research explanation, continuous stimuli input, nerve firing, and metabolism during activity might create too much noise for the repair process. However, this remains in the realm of hypothesis, and "what conditions make sleep conducive to repair?" will be the next research challenge.

2) Voices of agreement: Sleep might be more for "nerves" than "brains"

This aligns well with the sentiment that "sleep is primarily for maintenance." It involves processing accumulated damage and stress during wakefulness in a state with reduced sensory input. The human experience (feeling recovered after sleep) and the molecular-level explanation (DNA repair) seem to "connect."


3) The usual humor: Jellyfish nightmares, jabs at humans

As is typical on social media, jokes also appear. There are quips like "Do jellyfish have nightmares about failing grades?" and jabs like "There are humans without brains too," which are a "tradition" in this type of news. When scientific topics spread, the cycle of surprise, understanding, and humor is healthy. The lighter the entry point of interest, the easier it is for people to delve into the content (the relationship between DNA damage and sleep pressure).


So, has the "true nature of sleep" been resolved?

Rushing to conclusions can lead to misunderstandings. This study does not assert that "the sole purpose of sleep is DNA repair." Rather, it is reasonable to view DNA protection as "one of the oldest core functions" that has been reinforced. As the nervous system becomes more complex, other benefits like learning, memory, and synaptic adjustment may "accumulate later."


Nonetheless, the message from seemingly "primitive" organisms like jellyfish and sea anemones is strong.## Sleep may not have originated for consciousness or dreams but was first necessary "to protect fragile nerves."## And this necessity has been a "cost worth paying" for life for over 600 million years—thinking this way might make tonight's sleep look a little different.


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