system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do people get dark bags under their eyes when they’re tired?
gpt
So basically, you're so tired that you can't keep everything tightened up. As your muscles begin to weaken, the skin around your eyes droops. This causes it to contour to your natural wrinkles and facial structure. This forms the outside appearance of the darkened "bags". They appear swollen due to fat deposits around the eyes sinking, as well as a pooling of body fluids. Since the skin has sunk, it creates little pouches, which is what allows the little sacs of fat to drop down, normally the general tightness of the skin prevents this.