system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is happening when I'm so sleepy that my eyelids feel heavy?

You know when you're so tired that you can barely keep your eyes open and when you finally choose to close them, you get this sense of relief around your eyes and you quickly start dosing off,  and it just feels good. What exactly causes that feeling? And why is it that we have to close our eyes to fall asleep?
gpt
Your eyelids are raised by a muscle that is partially controlled by the part of your nervous system (the "sympathetic" nervous system) that is responsible for activating and energising your body (you may have heard of it in relation to the "fight or flight" response). When you get very tired that part of your nervous system is less active/dominant and so it is harder for you to raise your eyelids. The reason you experience this as "heaviness" is that you can still partially (consciously) control the muscle to try to open your eyes, but without the unconscious assistance of the sympathetic nervous system. You try consciously to open your eyes but you just don't have the usual, expected amount of power to work with.