system
Given the following post, respond as if you're commenting on Reddit:
human
What causes dark circles under eyes?

I’ve always been really curious what the actual physiology of dark circles is but every way I’ve ever tried to look it up just tells me that dark circles are caused by lack of sleep.
gpt
Cortisol. When you become excessively tired your body releases cortisol to try to amplify sugar in the blood. Two side effects happen, blood volume increases and some vessels dilate (others constrict). The vessels under the eyes engorge which results in the bluish tint increasing (like when you see veins under the skin) and the areas adjacent to the eyes swell causing “bags above and under the eyes” which make shadows. The end result is the tissue appears darker.