The mystery of sleep keeps getting deeper. A recent serendipitous discovery by a group of graduate students led them to prove that jellyfish seem to sleep. It’s a pretty remarkable discovery, not least because jellyfish don’t really have brains. It calls into question very purpose of sleep, which, despite years of study, is still not understood.

Sleep is dangerous for an organism; it’s a very vulnerable time. If sleep has persisted throughout evolution, there must be a good reason for it. Many have noted that intense physical activity does not automatically lead to sleep, however, so many modern hypotheses regarding sleep are focused on the brain.

Unfortunately, when it comes to studying the brain, technological prowess has outpaced understanding. We can image, read activity, and examine gene expression in the brain, learning neurologically and genetically what happens during sleep. But what is not the same as why. There are several basic theories, any or all of which might apply: Sleep may help regulate brain temperature, clear out toxins generated over the course of wakefulness, restore brain energy, physically restore brain tissue, aid learning and memory, or restore needed molecules to the brain and body that are used up in sleep.

Still, if sleep primarily benefits the brain, why does an organism without a brain sleep? The jellyfish findings mark the first time that sleep has been observed in an organism without a brain. Sleep has been studied in organisms with pretty simple brains, although studying sleep in simple animals is difficult since the electric impulses characteristic of sleep in mammals are not present.

The benefit of sleep seems to vary depending on the organism. The extremely simple roundworm C. elegans sleeps on a very weird schedule, and only before it reaches sexual maturity. The worm uses its nap time to grow new cuticle (outer layer) cells and restructure its body. Fruit flies, which are slightly more complicated, seem to require sleep for learning and memory, a function for which there is considerable evidence in mammals. Such findings raise the possibility that sleep does not serve a universal function across organisms, benefitting the brain or body in various ways.

Many researchers believe that while there may be different details in different organisms, there must be some common features of sleep regardless of what is sleeping. A growing consensus among those who hold this view is that sleep is needed for maintaining a healthy nervous system, not necessarily a brain. A more complex nervous system—with a brain—provides more opportunities for sleep to provide benefits. Even a brainless jellyfish has a nervous system, so there is no contradiction.

We still have a long way to go before fully understanding sleep. But if sleep can be understood in a jellyfish, that might be a step toward deeper knowledge.

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