What do scientists mean when they say “we don’t know why we need to sleep”?

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I see this a lot but isn’t the basic answer just that the brain needs to recharge and clean out toxins within the body?

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It depends on how far you’re willing to go before you accept the reason as true without needing further explanation.

If the answer is ‘because the brain needs sleep’ the next question is ‘why does the brain need sleep?’ and not being satisfied by ‘it just does’, there is a desire for an explanation of what sleep does to the brain. And then you will wonder why these processes can’t happen while being awake? What is it about sleep that allows this ‘recharging’ to happen, and what are we ‘recharging’ anyway? Why can’t ‘toxins’ (what are toxins?) be removed while we’re awake?

Why does the brain need to recharge? Why can’t it do that while we’re awake? The body regularly clears out toxins while we’re awake, so that can’t be the full story.

It’s unclear what exactly the brain needs to do when we’re asleep that it can’t do when we’re awake.

The pineal gland regulates circadian rhythm and much more.. If you look into the bodies processes (also the brain I’m using body as one whole system) you may gain more insight on this.

We know that during sleep the brain does a lot of stuff, like consolidating memories or clearing out certain waste products, but we don’t know why it can’t do that while we’re awake. We also don’t really understand why that’s crucial for life and why sleep deprivation can kill you.

What’s more, most animals need sleep. The need for sleep is so basic, that animals who cannot stop moving and/or being alert have developed something called unihemispheric sleep (only half of their brain sleeps at one time, while the other is awake). The fact that sleep is so pervasive is again quite strange and we don’t understand why so many animals need it and why no other type of mechanism which fulfills the same purpose has appeared throughout evolution.

The current top theory of why sleep has evolved as a necessity is:
1. In the past (and even now), organisms had periods where remaining active actually provided no survival benefit. Think to when an organism has just fed. Why continue producing energy and remaining active? If anything, it only puts the organism in unneeded danger.
2. Number 1 above led to organisms developing an “off” mode, where energy consumption was severely diminished. Over time, this evolved into what we now know as sleep.
3. The off time that this mode provided, naturally provided a good time to regenerate / perform many maintenance type tasks. These likely evolved simultaneously with sleep to occur at the same time.

Evolution explanations are always “just so” though, we don’t actually know this happened.