Even Brainless Jellyfish Sleep—and It May Explain Why You Need Rest Too
Quick Take
- Jellyfish and sea anemones enter true sleep-like states despite lacking brains or centralized nervous systems.
- Sleep appears to protect neurons by limiting or repairing DNA damage caused during periods of wakefulness.
- These findings suggest sleep evolved as an ancient cellular maintenance process, not just for memory or cognition.
Sleep is one of life’s most universal behaviors. Despite its ubiquity, it’s also one of the most mysterious. Humans spend about a third of their lives with their eyes closed and their brains in states far removed from wakefulness. For most of us, that works out to about 25 years of our lives spent sleeping. For centuries, scientists have puzzled over why sleep exists at all, especially given the risks it brings—reduced awareness of danger, fewer hours to find food or mates, and the vulnerability that comes with being unconscious. Shouldn’t natural selection favor those who require less sleep?
A new study published in Nature Communications sheds light on this question by examining sleep-like behavior in jellyfish and sea anemones—animals so simple they don’t even have brains. What researchers discovered not only challenges assumptions about how sleep evolved, but also highlights a fundamental biological necessity shared with humans: protecting neural DNA from damage.
What Was Observed?
Jellyfish and sea anemones don’t seem like creatures that would sleep. Jellyfish, for example, are gelatinous, brainless wanderers of the sea with a decentralized network of neurons rather than a single, structured brain. Still, researchers studying the upside-down jellyfish (Cassiopea andromeda) and the starlet sea anemone (Nematostella vectensis) found that both species enter periods with reduced activity and responsiveness—a state equivalent to what we would call sleep—for about one-third of each day, much like humans.
In the jellyfish, sleep-like periods occurred mostly at night, along with short midday naps. The sea anemones, in contrast, slept mainly during daylight hours. Despite these differences in timing, both species showed the hallmarks of sleep: a consolidated period of low activity, reduced responsiveness to stimuli, and an increased need for rest after being kept awake.
These observations were made by tracking behavior over time. Scientists noted changes in how frequently jellyfish pulsed their bells or how often the animals reacted to external cues, such as light or disturbances. Slower responses and lower activity rates signaled that the animals were in a sleep-like state, just as decreased alertness signals sleep in humans and other animals.
The Study’s Surprising Results
Sleep is nearly universal among animals with nervous systems, from insects and fish to birds and mammals, yet its fundamental purpose remains debated. Traditional hypotheses propose that sleep supports memory, energy conservation, or the removal of metabolic waste. It’s a fine theory if we’re just talking about humans, since these functions are tied to relatively complex brains. But how then would one explain creatures that lack a brain benefiting from sleep?
To answer that, scientists from Bar-Ilan University and collaborating institutions turned to two of the most ancient animals with nervous systems: jellyfish and sea anemones. Their goal was to understand whether sleep might have a more primitive, fundamental role—one that predates complex brains. By studying these organisms, researchers could trace sleep’s evolutionary roots and test whether the behavior served a basic cellular function as opposed to a cognitive one.
In both the lab and natural habitats, researchers monitored the jellyfish and anemones’ behavior using infrared video tracking and other observational tools. They documented patterns of activity and inactivity, identifying sleep-like behavior based on reduced movement and slower responses to stimuli. They also manipulated environmental conditions, using periods of forced wakefulness to gauge how the animals compensated afterward—a phenomenon called sleep rebound.
But the most novel part of the study involved measuring DNA damage in neurons that make up the nervous system, using indicators of this damage to see how being awake versus asleep affected the animals at a cellular level. They exposed the organisms to external stressors like ultraviolet radiation and chemical mutagens, which are known to damage DNA, to see if sleep behavior changed in response. And because melatonin regulates sleep in many animals, the researchers tested whether giving melatonin influenced sleep patterns and DNA damage in the subjects.
The results were striking:
- Both jellyfish and sea anemones slept about one-third of the day, just like humans.
- Sleep deprivation led to increased DNA damage in neurons.
- Exposing the animals to UV radiation or mutagens also increased neuronal DNA damage and triggered longer sleep periods afterward, indicating a recovery response.
- Conversely, melatonin—again, a hormone that promotes sleep in humans and many animals—increased sleep and reduced DNA damage in these organisms.
Put simply, the results suggest a two-way relationship between sleep and stress in nerve cells. When cells are active for too long, damage builds up, and the body becomes more driven to sleep. Sleep then provides a quieter period when that damage can be limited or repaired. Given this, rest appears to protect nerve cells. This is especially important because these cells do not easily replace themselves.
Okay, But What Does This Mean for Us?
You may be familiar with the metaphor that your brain is like an office, which over the course of the day gets messy and disheveled. Then, at night, while you sleep, a cleaning crew comes in and reorganizes everything. Sleep has long been viewed primarily through this lens: relating to higher brain functions—memory consolidation, cognitive restoration, and emotional regulation. While that may still be true for complex animals, the new findings suggest that the origin of sleep may lie in a much more basic biological imperative: preserving the integrity of neuronal DNA under stress.
The study implies that the need for sleep predates the evolution of centralized brains. Its roots may lie in early multicellular organisms that needed a way to cope with the cellular costs of being awake, such as DNA damage caused by environmental stress like sunlight, or the metabolic demands of continuous activity. Sleep, in this view, is an ancient maintenance behavior, essential for lifelong before complex cognition or learning existed.
Understanding sleep’s ancient origins reshapes how scientists think about this universal yet perplexing behavior. It suggests that even in humans, one of sleep’s core functions may be fundamental cellular protection, not just brain performance or mental health. While human sleep research often focuses on memory, emotional wellness, or disease, this study strengthens the idea that sleep’s benefits include guarding the DNA of neurons from damage that accumulates during wakefulness. Returning to the office metaphor, sleep not only organizes your filing cabinets but also tightens all the screws on the furniture.
That insight has real implications for public health. In humans, chronic sleep loss is associated with an increased risk of cognitive decline and neurodegenerative diseases such as Alzheimer’s and Parkinson’s, according to recent studies. If sleep truly plays a role in protecting neuronal DNA, regularly missing out on rest could allow subtle damage to accumulate over time, gradually affecting memory, thinking speed, and overall brain resilience. By clarifying how sleep protects brain cells at their most basic level, these findings may help guide future research into slowing or even preventing age-related brain decline.
An Ancient Survival Strategy
There’s something comforting about the idea that sleep isn’t just a modern luxury but an ancient survival tool—one shared with creatures drifting through shallow lagoons hundreds of millions of years ago. Long before dreams or alarm clocks, sleep may have existed simply to protect the most delicate cells we have. So the next time you cozy up in your pajamas and settle under the covers, remember that you’re not being lazy or indulgent; you’re participating in one of the oldest maintenance routines on Earth, giving your neurons the same gift jellyfish have relied on for ages: a little time to rest, reset, and stay intact.
