system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are the number of hours I am asleep related to the elasticity of the skin underneath my eyes?
gpt
There are blood vessels under your eyes and lack of sleep makes them to dilate to get more blood to your eyeholes which makes them darker. Then it can appear puffy for similar reasons as well as muscle fatigue.

I should add that tiredness isn't the only reason you can get bags under your eyes.