system
Given the following post, respond as if you're commenting on Reddit:
human
If the default position for our eyelids is open, why do our eyes "get heavy," and why does it feel so good to close them when we're sleepy or tired?
gpt
So the default isn't open. It actually takes some strength to keep your upper eyelid from drooping over your eye, to keep your eyes open. That's why you notice it when sleepy. Some muscle diseases also have the symptom that eyelids start sagging early during the day, just because the muscles are too weak.