An upside-down jellyfish drifts gently in a shallow lagoon. At first glance, it looks like it’s just floating, but scientists have discovered something quietly astonishing - around midday, the jellyfish takes a brief nap to recover from a disturbed night.

The fact that jellyfish, which do not have a brain, are sleeping is amazing, and that discovery may completely reshape how we understand why all animals, including humans, need sleep.

A new study published in Nature Communications tracked sleep-like behaviour in two simple sea creatures - the upside-down jellyfish (Cassiopea) and the starlet sea anemone (Nematostella).

Both belong to an ancient lineage that dates back more than 500 million years, long before brains, backbones, or even eyes evolved.

Despite their simplicity, these animals clearly cycle between periods of activity and rest. When they’re resting, they respond more slowly to food or flashes of light, a key biological hallmark of sleep seen across the animal kingdom.

For decades, scientists have debated what sleep is actually for.

Is it about saving energy?

Consolidating memories?

Clearing out metabolic waste?

In humans and other vertebrates, sleep is known to help repair damage in the brain.  

While we’re awake, tiny breaks form in DNA inside our neurons, during sleep, repair mechanisms kick in and fix that damage.

Until now, scientists thought this kind of 'neural housekeeping' required a complex brain, but the jellyfish just proved otherwise. 

The researchers found that while jellyfish and sea anemones are active, DNA damage builds up in their nerve cells -when they rest, that damage is repaired.

When scientists deliberately kept the animals awake by altering water currents, the DNA damage increased and so did the amount of sleep the animals needed afterward, mirroring the 'sleep rebound' humans experience after a bad night. 

When researchers exposed the animals to UV light (which damages DNA), they slept more. Once they’d rested, the damage decreased and their normal rhythms returned.

Even more surprising? Adding melatonin, the hormone many humans take for jet lag caused these brainless creatures to fall asleep at unusual times.

That suggests melatonin’s sleep-inducing role evolved far earlier than scientists thought.

Put together, the findings point to a powerful idea: sleep may have started as a cellular defence mechanism.

Being awake is stressful for nerve cells as sensory input, movement, and metabolism all create molecular wear and tear. Sleep creates a quiet window of reduced stimulation where essential repairs can happen safely.

If even jellyfish need that repair time, then sleep likely evolved before brains, before complex nervous systems, and before animals even had left and right sides.

This doesn’t mean sleep has only one purpose.

In animals with more complex brains, functions like memory consolidation and learning likely layered on top of this ancient repair role, but the study strengthens the idea that protecting DNA may be the most fundamental reason we sleep. 

The study offers a sobering perspective on modern life, as chronic sleep deprivation in humans has already been linked to neurodegeneration and increased DNA damage. If creatures as simple as jellyfish can’t function without sleep, it’s a reminder that rest isn’t a luxury, it’s a biological necessity baked into life itself. 

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