system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What causes us to get bags/dark circles under our eyes when we are tired?
gpt
Dark circles are just the blood that is circulating around your eyes showing through the skin. The skin under the eye is very thin, so subtle changes can cause these circles to appear. In essence, there are three main reasons why this can happen:

* your blood vessels dilating (low blood pressure). this can be because of inflammation such as with allergies or rubbing. when you have an inflammatory reaction, your body releases histamine, which causes a reaction to get as much of your immune cells to the area as possible. this will cause more blood flow, resulting in a lower blood pressure and dilated blood vessels. this means more of them show through the skin. it can also be because of heart medication or erectile dysfunction drugs which work by causing veins to expand. being tired will cause dryness and irritation around your eyes, which will lead to an inflammation similar to an allergic reaction, causing more of the darkness to appear, especially if you irritate the area more by also rubbing it.
* your skin getting pale. if you are not getting enough sleep, anemic, or dehydrated, your skin will become paler and that will show more of your blood vessels through your eyes.
* your skin getting thinner. this is mostly the cause of dark circles when you age.

Bags are caused by areas under your eyes filling up with water or fat. This is essentially a secondary effect of inflammation. Expanded blood vessels will cause fluid to accumulate and pool, and leak out of your blood. This is normally beneficial because the fluid will have immune cells in it that will help fight an infection or irritant. This is why injured areas will be red, hot, and swollen, but in this case it will cause these bags. In addition, rubbing your skin (as is common when being irritated) will exacerbate this, essentially mechanically squeezing even more fluids out of the blood vessel.