system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does sleep restore the body's energy?

If energy can't be created, then why does sleep boost us back to 100% every morning? If I don't plug in a laptop and I put it in sleep mode, it just drains battery less quickly. If it's from food, why does our body wait until sleep to convert it to energy?
gpt
It's from food, and the body doesn't wait until sleep to restore energy.  This is why a workout or athletic activity feels so much easier a few hours after a meal rather than on a starving stomach.

What sleep restores is not your energy, but your brain function.  Your brain is floating in a nutrient soup called *cerebrospinal fluid* or CSF.  Throughout the day, your brain cells drink in nutrients from the CSF, and spit out waste.  This builds up, and by the end of the day there's so much waste floating around in the CSF that it starts negatively affecting brain function.

When you sleep, channels open up and flush the day's supply of CSF away.  New, clean CSF is pumped in for tomorrow, and so you wake up with a clean brain.