An upside-down jellyfish drifts in a shallow lagoon, rhythmically contracting its translucent bell. By night that beat drops from roughly 36 pulses a minute to nearer 30, and the animal slips into a state that, despite its lack of a brain, resembles sleep.

Field cameras show it even takes a brief siesta around noon, to “catch up” after a disturbed night.

A new Nature Communications study has tracked these lulls in cassiopea jellyfish, which belong to a 500 million year-old lineage, as well as in the starlet sea anemone nematostella. The study findings may help settle a long-running debate among biologists about what sleep is for.

Does sleep conserve energy, consolidate memories – or do something more biologically fundamental? Until recently, most evidence for a “house-keeping” role for sleep came only from vertebrates.

When mice sleep, brain and spinal cord fluid surges through the brain and washes away metabolic waste. And a 2016 mouse study found that some types of DNA breaks are mended more quickly during sleep. Time-lapse imaging in a 2019 study of zebrafish showed that sleep lets neurons (nerve cells) repair DNA breaks that build up during waking hours.

The new study showed for the first time that the same process occurs in some invertebrates. That while the jellyfish and sea anemone are awake, DNA damage accumulates in their nerve cells and when they doze, that damage is........

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