The Deep Evolutionary Roots of Sleep

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Ssleeping is risky. When you fall asleep, your defenses are weakened, making you an easy target for predators and other threats. And yet the majority of living things do it, including most animals, insects and even many plants.

Sleep, of course, has many documented benefits: saving energy, repairing the brain, tracing memory. But its original function is more difficult to define. Why did it first evolve? It’s a mystery that scientists have been trying to solve for decades.

Recently, a team of Israeli scientists decided to probe the ocean in search of an answer, placing two ancient sea creatures in their field of vision: the lyrically named upside-down jellyfish (Andromeda Cassiopeia) and the starlet sea anemone (Nematostella vectensis). Jellyfish and sea anemones are among the oldest animals to have evolved on Earth and belong to the phylum Cnidaria, whose fossils date back to the Precambrian period, approximately 580 million years ago.

Publish their findings in the journal Natural communicationsScientists report that these early creatures needed as much sleep as humans and that sleep may have originally emerged as a way to protect vulnerable neural tissue from the ravages of the daily grind.

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Scientists already knew that these two inhabitants of shallow coastal waters have very different shutdown schedules: The jellyfish sleeps at night, with occasional naps during the day, while the latter is most active during twilight hours, at dusk and dawn. But little was known about their specific sleeping habits.

The starlet sea anemone is a small, mostly translucent burrowing creature, with colorful, frilly tentacles, while the upside-down jellyfish rests belly down on the seafloor, its arms branching above like an underwater flower. These animals are so simple that they have no brains and instead rely on networks of neurons called neural networks to help them coordinate their activities.

First, Israeli scientists had to establish what counts as sleep for each creature. A jellyfish’s tentacles never stop beating, so how do you know when it’s dozing? Looking for attenuated pulses over a prolonged period and slowed reaction times to light, they noted that jellyfish sleep could be described as less than 37 pulses per minute for a 3-minute period. For the starlet sea anemone, they used an infrared camera and a light stimulus to identify the dividing line between nap and wakefulness, determining that these animals sleep when they have been lazy for 8 minutes and show relative indifference to light and food.

Using these definitions, the scientists reported that both creatures slept about a third of the day, or about 8 hours in human terms, in their natural habitat, and that their sleep schedules responded strongly to sleep deprivation, ambient light, and circadian rhythms. Feeding the creatures melatonin – a natural hormone involved in regulating sleep, the production of which is influenced by light – led to increased sleep during periods when each would normally have been more active, but had little impact on their activity during normal resting phases. It also reduced DNA damage in both species at times of day when they were normally alert.

The scientists also measured the DNA damage of the two creatures under different conditions and found that it increased when they were awake and was amplified when they were sleep deprived, but decreased when they were asleep and after “recovery” sleep.

Most surprisingly, they found that DNA damage caused the creatures to sleep more. In jellyfish and sea anemones, scientists have caused damage by exposing them to certain chemicals and certain frequencies of light. Then, they recorded the sequences of events that followed: first, additional DNA damage accumulated, then the creatures slept longer, after which the DNA damage decreased. The relationship seems to work both ways: more DNA damage leads to more sleep, and more sleep reduces DNA damage.

The results suggest that sleep may have come into play to allow living things to survive the ravages of time.

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Main image: Minakryn Ruslan / Shutterstock

• Kristen French

  Published on January 7, 2026

  Kristen French is an associate editor at Nautilus. She has worked in science journalism since 2013, reporting and writing articles and news for publications such as Wired, Backchannel, The Verge, And New York Review. She holds a master’s degree in science journalism from Columbia University.