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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 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 stick close to the surface of the water so that 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 a 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 also 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 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 any 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've 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.

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, in turn, could 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.

And with a 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 in the description will get 25% off a premium Fabulous subscription.

That helps us out too, so, thank you.