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You might think you need a brain to be able to sleep, but organisms with super simple neural networks can still "sleep" sort of like we do. So, if these organisms can sleep too, then what is sleep anyway?

Hosted by: Stefan Chin

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This episode is sponsored by  Fabulous, an app that helps you form healthy habits that stick.

Click the link in the description to  get a free week trial and 25% off a Fabulous premium subscription! [♪ INTRO]. Let me ask you a question.

What  came first, sleep or the brain? Many would probably answer that  question with brain, obviously. Except, recent research points out that  sleep might not be as connected to the brain as scientists once thought.

Because organisms with super  simple neural networks can still sleep sort of like us humans. So, if these organisms can “sleep”  then what is sleep, anyways?   Well there are lots of creatures  that need to sleep, but they don’t all approach it in the same fashion.   For example, humans usually  sleep for several hours in a row, with teenagers being the  ones who sleep the most. Wild elephants, though, only sleep for a  couple of hours a day, sometimes going days without entering a  deeper, more restful type of sleep.

So what’s the brain doing while  these creatures are snoozing? Researchers can measure the electromagnetic  signals coming from the brain to get an understanding of what’s  happening in there during sleep. Let’s take dolphins, for example.

Their way of sleeping is known as  unihemispheric, slow-wave sleep. Which means they sleep with half of their brain. When researchers measured the  electromagnetic signals coming from dolphin brains, the signals  coming from each side of the brain were completely different.

Half of the brain was in  slow-wave sleep, while the other side of the brain  showed signs of wakefulness. But we can also easily observe other, more  outward signs that dolphins are asleep. For example, they might close one eye, the eye opposite the brain hemisphere that’s asleep.

And they may also stick close  to the surface of the water, so they’re able to easily surface to breathe. And studies investigating sleep in invertebrates, like fruit flies and cockroaches,  found that these creatures also do things that signal that they’re asleep. These include a decrease in  their behavior and responsiveness as well as change their body posture.

So factoring in the different  ways creatures sleep, plus the idea that there are these  common tell-tale signs that a creature is sleeping, researchers began to look  for sleep all over the tree of life. And they found evidence in a lot of organisms, including some very simplistic  creatures, like hydras and jellyfish.   Compared to us humans, their neurons  aren’t as densely packed together. They’re more like a light  mesh of neurons than brains.

By studying this mesh, researchers  can tell more about what the first sleeping creature was like  and what it was using sleep for. A potential clue for that first  sleeping critter is jellyfish, which can enter that sleep state,  leading scientists to believe that sleep evolved more than one billion years ago! And finding where in the tree of life  organisms like jellyfish converge with mammals can help researchers  uncover the incredibly ancient common ancestor that we share.

Now for creatures with only  a sparse mesh of neurons, researchers think that sleep probably  plays a role in their metabolism, the amount of energy a creature’s  body uses to maintain itself. So for organisms that have  a simplistic neural network, their bodies are just changing what  they do with the available energy. Entering a state of sleep may  trigger reactions to occur that can’t happen while the creature is awake.

Or sleep may just provide enough  available energy for these reactions to take place. For example, the nematode C. elegans uses the time it’s asleep to  grow and repair its tissues. ​​This creature doesn’t sleep  at regular intervals each day. Instead, it only sleeps  after periods of development.

And researchers have also  found that sleep-deprived hydras pause the daily  division of their body cells. Recent research links metabolism  and sleep in organisms with more complex neural networks, like humans. So, sleep is tightly woven into the  human body's hormonal and metabolic processes and is vital in keeping  the metabolism functioning properly.

Which means that if you’re  sleep deprived or have a sleep disorder it may negatively  impact the body’s metabolism. So, animals from humans to  critters with just a mesh of neurons can at least sort of sleep, but  what if you don’t have neurons at all? That ancient common ancestor between  jellyfish and mammals probably had neurons that would have transmitted  a signal to the muscles to cause the organism to move.

And when it wasn’t moving, it would have  been considered to be in a sleep state, like how we can measure the outward  behavior of sleeping dolphins, elephants, or humans. But the jury is still out on creatures lacking neurons and muscles altogether,  like sponges for example. But measuring something like this has  proven to be a challenge, because there’s no electrical signals to  detect, and the sponge doesn’t really move in the same way that other animals do.

So researchers can’t look for a change  in body posture as an indicator of sleep. Sponges do have a metabolism,  they are, after all, a living, breathing creature that has energy demands. Researchers just haven’t yet come up with  a way to tell whether these creatures go through a metabolic cycle,  pausing some of their activity to use that energy for other  things, on a cellular level.

A better understanding of  whether these kinds of creatures sleep could help answer many  questions surrounding sleep. And this could, in turn, help researchers  better understand human sleep, potentially leading to new ways of  treating sleep-related diseases, or the development of new drugs  that target spots in the body previously thought to be completely  separate from the sleep process. So, thanks to some of the more  simplistic creatures on this planet, scientists have learned that sleep is anything but a simple, one size fits all process.

And something else that could  help you keep tabs on your sleep is today’s sponsor Fabulous! They’re a self-care and habit  forming app developed at. Duke University’s Center for  Advanced Hindsight and they have over 20 million users.

The app is customizable to support your  personal goals, like crafting your space to be distraction-free or setting a realistic  bedtime goal to have a good night’s sleep. With Fabulous premium subscription,  you also unlock coach sessions and. Journeys to inspire a shift in mindset, like incorporating small simple  habits in your evening routine.

If you wanna check them out, the first 100  people who click on the link will get a free week trial and 25% off  a premium Fabulous subscription! That helps us out too, so thank you. [♪ OUTRO].