The body needs time to focus on certain maintenance tasks for which there was no time during the day. Muscles recover, food is digested, the brain is dreaming to help process new information etc.

This is because it was important for our ancestors to completely focus on activities such as hunting while awake and use sleep to recover at night while it is too dark to do much anyway.

We're not SUPER sure exactly, but it's kind of like how your phone needs to turn off to update. I would imagine your brain rewiring things significantly while you're awake would get REALLY weird.

But why this recovery cannot be done while awake

It's like getting an oil change while the car is still running and being driven. The oil that is in circulation is at a high temperature so it's inefficient for lubrication and movement. The engine components are also moving at a high speed so any opening of the oil tank could cause a disruption to the movement of the car and could potentially cause damage. The same is true for sleep, while you are awake, your body is in a constant state of motion and as a result cannot receive the full benefits of quality rest and healing. To get the full restoring benefits, your body needs to power down and go into a restful state.

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The resources are needed for the actual activities.

Like you cannot defragment your harddrive while playing a game, or more general you cannot update the firmware of a device while using it.

It could theoretically be done, but we have evolutionary evolved using sleep as we know it as a recovery strategy. It has shown to be an advantage.

Birds for example also have sleep, but they evolved in a way that only one half of the brain sleeps at any time because that allows them to keep flying while recovering which turned out to be an advantage.

Digesting food is a big one. You essentially have enough blood (ability to distribute oxygen and calories) to run two of three between your stomach, your brain, and your muscles. One of the reasons working out while on a full stomach causes vomiting is because the stomach is upset about the lack of resources available for it to do its job, so it gives up to protect you from harm.

Alternatively, eating after a long day of work puts you straight to sleep because your muscles need to heal and clean, and your stomach needs to digest dinner. You can't run your whole body at once, and sleep is what we call it when your brain and muscles are turned off to let cleaning, healing, maintaining, ad digesting to happen.

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