system
Given the following post, respond as if you're commenting on Reddit:
human
What causes the area around our eyes to turn dark when we are tired?
gpt
iirc A doctor answered this question a while back:

The skin under your eyes is one of the thinnest areas in your body.  Therefore small changes in the blood are most noticeable there.  In the case of bags under the eyes it is due to increased adrenaline flow in an attempt to keep you awake.

Not an exact quote but should answer your question.