system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we get "bags" under our eyes?

I'm talking in the sense of dark skin color under the eyes, rather than old age and skin sagging.
gpt
Lack of sleep makes skin more pale.

Skin more pale means it contrasts with the dark areas.

In other words, your skin near your eyes is always dark (the skin near your eyes is a little bit see through, and you can see the darkness of the blood behind it).  However, when your skin is pale due to lack of sleep, it that see-through darkness seems even darker compared to your lighter skin, so it looks like you have dark circles there when you don't sleep enough.