Brainless Jellyfish Sleep Like Humans - It Reveals Why Sleep Evolved

New research reveals that even brainless marine creatures like jellyfish and sea anemones experience sleep, offering clues to one of biology’s oldest mysteries: why we sleep.

According to a study published in Nature Communications, the upside-down jellyfish (Cassiopea andromeda) and starlet sea anemone (Nematostella vectensis) enter daily sleep-like states lasting about eight hours—just like humans. The finding suggests that sleep evolved not with complex brains, but with the very first neurons.

Neurons are very precious, says Dr. Lior Appelbaum, molecular neuroscientist at Bar-Ilan University and co-author. They don’t divide, so you need to keep them intact. (Source from Nature news By Lynne Peeples.)

What Is Sleep—And Why Do We Need It?

Sleep is risky. It leaves animals vulnerable. Yet it persists across species—from flies to whales—implying a vital function.

Scientists have long suspected sleep plays a role in cellular repair. This study confirms it: wakefulness causes DNA damage in neurons, and sleep helps repair it.

In both jellyfish and anemones, researchers found DNA damage builds up during waking hours. Sleep, whether natural or induced by melatonin, reduced this damage.

Even more striking: exposing the animals to UV radiation or DNA-damaging chemicals increased their sleep pressure—proving a direct link between DNA damage and the need to sleep.

Sleep Without a Brain: How Is It Possible?

Neither species has a brain. Instead, they have simple nerve nets—diffuse networks of neurons.

Yet they meet all behavioral criteria for sleep: periods of immobility, reduced response to stimuli, and recovery after sleep deprivation.

The jellyfish slept at night and took midday naps. The anemone, active at dusk, slept more at dawn—regulated by its internal clock.

Every time somebody adds to the list of species that sleep, it is a very important step for the field, says Chiara Cirelli, sleep researcher at the University of Wisconsin–Madison, not involved in the study.

A Universal Function: Genome Protection

The study suggests sleep’s core evolutionary purpose is to protect neuronal DNA.

Since neurons don’t regenerate, they must be maintained. Sleep provides a “consolidated period” for repair—free from the metabolic and environmental stresses of wakefulness.

Melatonin, a hormone tied to sleep in humans, also induced sleep in both species—despite their different activity patterns—hinting at an ancient, shared sleep-regulating mechanism.

Why This Matters

Understanding sleep in these ancient animals helps trace its origins to over 600 million years ago—possibly predating centralized nervous systems.

It also reinforces that sleep isn’t about consciousness or cognition—it’s cellular maintenance.

Future studies may explore whether coral colonies, made of interconnected polyps, exhibit synchronized or rotating sleep patterns.

The Bottom Line

Sleep likely evolved not for dreaming, learning, or memory—but to repair DNA in irreplaceable neurons.

As Appelbaum puts it: "Sleep may have emerged with neurons themselves."

This discovery reshapes our understanding of one of life’s most essential behaviors.

This is a developing story in evolutionary neuroscience.