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.
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.
Earlier evolving creatures, with fewer cell types, less complicated molecular pathways, and simpler behaviors may reveal sleep in its most fundamental form. Gleaning insights into its basic function from those species could be difficult. If that’s true, sleep in humans, rodents, and other vertebrates is a highly evolved behavior-one adapted to each organism’s needs and lifestyle. “I think we didn’t evolve sleep, we evolved wakefulness.” The earliest life forms were unresponsive until they evolved ways to react to their environment, he suggests, and sleep is a return to the default state. “I think if it’s alive, it sleeps,” says Paul Shaw, a neuroscientist from Washington University in St. Calling the restful, unresponsive state seen in jellyfish and hydra “sleeplike” is more acceptable to him.īut others in the field are pushing for a much more inclusive view: that sleep evolved not with modern vertebrates as previously assumed, but perhaps a half-billion years ago when the first animals appeared. “I do not believe that many of these organisms sleep-at least not the way you and I do,” says John Hogenesch, a genome biologist at Cincinnati Children’s Hospital Medical Center. That makes studying sleep in brainless creatures controversial. Scientists have often defined sleep as temporary loss of consciousness, orchestrated by the brain and for the brain’s benefit. To catch one snoozing could upend researchers’ definition of sleep and their understanding of its purpose. Sponges, some of the earliest animals to appear on Earth, fit that description. Now, “The real frontier is finding an animal that sleeps that doesn’t have neurons at all,” says David Raizen, a neurologist at the University of Pennsylvania (UPenn) Perelman School of Medicine.
Over the past few years, studies in worms, jellyfish, and hydra have challenged the long-standing idea that sleep is unique to creatures with brains. That’s not as silly a question as it seems. No researchers paid them much mind until 2017, when William Joiner, a neuroscientist at the University of California (UC), San Diego, decided to look into whether sponges take naps. A version of this story appeared in Science, Vol 374, Issue 6567.ĭive among the kelp forests of the Southern California coast and you may spot orange puffball sponges ( Tethya californiana)-creatures that look like the miniature pumpkins used for pies.
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