system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do I have dark rings under my eyes?

What causes this?  Why are they dark?
gpt
This is pathetic, guys. One link, a suggestion to google, a bunch of jokes? It's a legitimate question.

Under your skin, there are a lot of things that carry blood through your body--blood vessels. Usually, your skin is thick enough that you don't see these, but it so happens that the skin is very thin right under the eyelids. You can see through it a little bit, and that's why it looks dark (remember that blood doesn't necessarily look red unless it's out in the open).

You might wonder why some people have this and others don't. Most people inherit dark circles through their genes--it's passed down. Sometimes dark circles are a sign of other things, like anemia or liver issues. They are associated with sleepiness because fatigue (being tired) causes you skin to get paler, which makes it easier to see the dark blood beneath it. Usually, it doesn't mean anything bad, and you shouldn't worry unless there are a lot of other bad things happening.

The skin around your eyes is called the "periorbital" region, if you want to look up more.

**TL;DR: Skin around your eyes is thin, which means you can see blood vessels underneath it.**