You're probably thinking of the so-called "glymphatic system" that was in the news a few years back. https://en.m.wikipedia.org/wiki/Glymphatic_system

Yep, that's exactly the one I was thinking of. Thank you!

See also https://en.wikipedia.org/wiki/Cerebrospinal_fluid#Circulatio...

with a nice MRI of fluid being pumped around. Though I think a different system to the one in this article.

> being one of the reasons we need sleep

Jellyfish have no brain, but also sleep. So that can't be the reason. (It might happen then anyway, but it's not the fundamental reason.)

You guys keep saying that silly myth.

They don't have a BRAIN but they have nervous system. A nervous system creates waste and that waste needs to be reclaimed. What's a brain? An exotic collection of neurons... that create waste.

http://neurodojo.blogspot.com/2009/05/jellyfish-nervous-syst...

>Instead, jellyfish have a ring nervous system, located along the margin of the bell. There is definitely a concentration of neurons in that location (although it contains relatively few neurons compared to other animals). Plus, those neurons do serve as an active relay and processing station for sensory and motor activity. Those are two of the main things that central nervous systems do, so there seems to be no good reason to deny that jellyfish have a central nervous system. (Picture from Mackie and Meech, 2000.)

https://en.wikipedia.org/wiki/List_of_animals_by_number_of_n...

Jellyfish: 5,600 neurons.

Box jellyfish: 8,700-17,500 neurons.

Human: 86,000,000,000 neurons.

Notice jellyfish do not have "zero" neurons.

Further, from the wiki: "Not all animals have neurons; Trichoplax and sponges lack nerve cells altogether."

https://www.quora.com/Do-sponges-sleep

"Do sponges sleep?"

"As noted by others, sponges lack nervous systems, [...] sponges don't have nerve cells at all. So NO they don't 'sleep" in any manner similar to other animals."

It could also be that they don't sleep in the sense of humans.

This seems to have flown over your head, but humans fall into that "other animals" category

Although I appreciate this kind of reasoning in general, I think it can be misleading when applied to animal behavior.

What prevents one of the reasons we sleep to be absent from the set of reasons jellyfish sleep?

Also, wouldn't it be surprising if a behavior as complex as sleeping had only one reason, or if the main reason today would be anything like the main reason back in the days were the common ancestor between us and jellyfish lived? And so, shouldn't we expect that we sleep for different reasons?

The quote says "one of the reasons". You replied "can't be the reason". There's a fundamental disconnect there: the person you're replying to didn't say that there was any fundamental reason, or that they were describing it.

Jellyfish have 5600 neurons though.

5600 neurons ought be enough for any species.

--- Bill Gates 1982mya

I thought they did have a brain, it was just spread out through their whole body.

We don’t need to sleep, but our body will put us to sleep. There is no clear answer as to why we sleep. But one acceptable theory is our brain “understands” sleeping is going to help functioning. During sleep our immune system finally has the opportunity to clean up waste. The 2017 Nobel Prize winners for Medicine and Physiology discovered there are genes famously work as “inner clock”, and they think these genes are responsible for all the routines in our body. But fundamentally we still do not have an absolute answer to why we sleep, just like we have several theories on why we dream. However, I believe whatever theories exist for why we sleep and why we dream, they are union of a bigger theory, not mutually exclusive.

[1]: https://www.google.com/amp/s/amp.theguardian.com/science/201...

[2]: https://www.nobelprize.org/nobel_prizes/medicine/laureates/2...