Jellyfish and sea anemones are curious creatures: these organisms evolved without a brain and, as scientists discovered in the past several years, don’t need one to sleep. The animals do, however, have neurons—nerve cells that appear interconnected throughout their body. A new study shows that how these animals sleep is surprisingly similar to the way humans do, suggesting that humanlike sleep might have evolved before even the most primitive brains.
The findings, published in Nature Communications, also shed light on one of science’s prevailing mysteries: Why do animals sleep? They add to past evidence from other animals and humans that sleep provides a “window” of downtime for maintenance on the brain and body, helping to repair DNA damage and maintain neuronal health.
“This confirms that sleep allows a window for key housekeeping tasks,” says Philippe Mourrain, an associate professor of psychiatry and behavioral sciences at Stanford University who studies sleep and was not involved in the new study.
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The work shows this function of sleep has been conserved across evolution, Mourrain says, from animals with some of the most complex brains, such as primates, to animals that have none, such as jellyfish and other cnidarians.
The researchers studied one jellyfish and one sea anemone species, finding that the jellyfish appears to enter a sleeplike state for around eight hours a day and generally at night—a schedule many humans might recognize. The sea anemone analyzed also appears to shut down for around a third of the day.
The study animals slept more when the researchers damaged their neurons, a finding that Mourrain says gives clues to what makes us fall asleep in the first place.