system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What exactly about sleep “recharges” the body and gives us energy?
gpt
There's a chemical called Adenosine that builds up in your brain throughout the day. When it reaches a certain level, it makes you feel sleepy. When you sleep, your brain removes the built up adenosine.

I highly recommend the book "Why We Sleep" by Matthew Walker. It is full of useful info like this and tons more.