system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do tired eyes look sunken and dark?

Like the skin under our eyes sometimes look sunken in and dark or purple when we are very tired.
gpt
When you get tired, certain hormones that restrict blood, and keep you awake, like epinephrine and cortisol are reduced in your blood, and that can cause blood to accumulate in the eye area, the skin on/around your eyelids is much thinner than most of the rest of your body, so the blood can cause puffiness and show up dark in that area.