system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why/How do organisms wake up from sleep?

What happens when you sleep that wakes people up?
gpt
Sleepiness is caused by a certain little kind of chemical in your brain attaching to neurons. That slows your nervous system down, and you feel like sleeping. While you sleep, your body stops producing that little chemical, and the little chemical slowly dissipates from your brain, allowing you to feel awake again.