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THE OCTOBER 29, 2021, SCIENCE magazine is a special issue focusing on “Why We Sleep.” Not only “we,” but a surprising number of other living creatures, even to the most elementary ones. Here are tidbits about this almost universal topic.

Sleep Basics. In “The Simplest of Slumbers,” Elizabeth Pennisi writes, “By the 1950s and ’60s, researchers were converging on a definition of sleep based on polysomnography, a combined measure of brain activity, eye movement, and muscle tone that became the gold standard.” Human sleep has two major stages: rapid eye movement (REM), a more active stage in which dreaming occurs; and non-REM, defined by slow, synchronous waves of electrical firing.

Human sleep, Pennisi notes, “helps the brain consolidate memories and flush out toxic wastes. It may also help the brain stay plastic by pruning and strengthening connections between nerve cells.” 

But What About Beings Without Brains? Pennisi quotes neurologist David Raizen, “The real frontier is finding an animal that sleeps that doesn’t have neurons at all.” 

Pennisi writes, “So, some sleep researchers have turned to invertebrates such as fruit flies and roundworms—and most recently to sponges and another early-evolving group, placozoans. Already, their work is driving home two key new insights: that sleep’s benefits extend far beyond the brain, and that muscles, the immune system, and the gut can all have a say in when and how sleep occurs.”

Cassiopea, the upside-down jellyfish. Image from Science, October 29, 2021.

The upside-down jellyfish, Pennini observes, “tends to stay near the shallow sea floor, pulsing with its tentacles pointing up so more light reaches the photosynthetic microorganisms it relies on for energy.” 

Researchers have observed that, at night, this motion slowed from 60 pulses per minute to 30. What’s more, Pennisi writes, “… the drug melatonin, an over-the-counter sleep remedy, slowed their pulse to nighttime speeds. All this without a real brain: Jellyfish have a ring of nerve cell clusters around the rim of the bells.”

Placozoans—As Basic As They Come. “Placozoans,” writes Pennisi, “are round, flat, transparent creatures no bigger than a sesame seed that have just two layers of cells, each outfitted with whiplike projections called cilia.”

“Placozoans lack nerve cells;” Pennisi says, “their cells communicate via chemical secretions that control cilia movements.” Researchers say that, apart from parasites, placozoans are the simplest animals on Earth.

Placozoans crawl randomly along the tideline as they seek out microalgae. However, researchers have found that even placozoans slow down at night. In a sense, it’s a first evolutionary step toward sleep, a rhythm of rest to recharge for the next feeding cycle.

The Signs of Sleep. Among human characteristics of sleep are physical quiescence, posturing, changes of brain or cellular activity, and following circadian regulation. A goodly number of living creatures share a goodly number of these behavioral characteristics.

Our Sleep, in Context. First, it’s clear from recent research that humans are not unique among those benefiting from sleep. Second, in “A Translational Neuroscience of Sleep: A Contextual Framework,” Michael A. Grandner and Fabian-Xosé Fernandez write that “Sleep is a non-negotiable biological state required for the maintenance of human life…. our need for sleep parallels those for air, food, and water.” 

From the paper’s Abstract: “Sleep is entwined across many physiologic processes in the brain and periphery, thereby exerting tremendous influence on our well-being. Yet sleep exists in a social-environmental context.”


Among other aspects, the researchers cite our 24/7 society, our neighborhoods, our families, and our social networks.   

Yes, something to sleep on. ds 

© Dennis Simanaitis,, 2021

2 comments on “LET’S SLEEP ON IT

  1. Jack Albrecht
    November 16, 2021

    Very interesting. Did anyone else yawn while reading about sleeping?

    • simanaitissays
      November 16, 2021

      Ha. I once worked with a woman who was a “sympathetic yawner.” The fakiest yawn by anyone near her would evoke an involuntary yawn on her part.

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