All about Dark Circles

How and why your body creates dark under-eye circles

Learn about the causes of dark circles under eyes
Most people think that dark under-eye circles are caused by staying up late watching monster movies, or having that last drink the night before, or sitting up with your laptop trying to finish the quarterly report. Most people think that their behavior is somehow linked to dark under-eye circles.
Well, most people are dead wrong.
If nothing else, I want you to know that dark under-eye circles are not your fault. They don't mysteriously appear when you misbehave or are stressed out, only to vanish when you get 8 straight hours of sleep. Dark under-eye circles are a by-product of the very same mechanism that produces bruises (and you don't blame yourself for bruising, do you?). Changing your behavior will, in almost every case, NOT get rid of your dark under-eye circles.
Having said that, onto the specifics:

What causes dark under-eye circles

Your shiners, your raccoon eyes, your blue luggage... whatever you call your dark under-eye circles, here's what they really are: oxidizing hemoglobin.
Dark under-eye circles begin in the capillaries, the tiny blood vessels that web the delicate skin around the eyes. Now, your capillaries are so small that red blood cells sometimes have to line up, single file, to get through. Frequently, red blood cells get lost and wander into the surrounding skin. This isn't a problem - it happens all the time - and your body has a mechanism to mop up these escapees. Enzymes in your body break down the red blood cells, including their hemoglobin (the molecule that gives them their distinctive red color).
No problem, right? Except for one thing: when hemoglobin is broken down, its remaining components have a dark blue-black color. Just like a bruise. So your dark under-eye circles are actually caused by leaky capillaries.

How dark under-eye circles are like bruises

When something hits you, blood vessels are traumatized and sometimes broken. Blood leaks out into the surrounding skin. Your body begins the mopping-up process, and you see a dark, purplish or blue-black discoloration.
So, as you can see, dark under-eye circles are very similar to bruises. The same mechanisms produce them.

Why are dark under-eye circles so visible?

It's quite possible that capillaries all over your body are leaking small amounts of blood all the time (I don't know if this is true or not). But the reason dark under-eye circles are so apparent is this: the skin around the eyes is some of the thinnest, most delicate skin of your entire body. The capillaries are much closer to the surface of the skin there. Many people's skin is not only thinner around the eyes, but also more translucent.
The combination of capillaries near the skin's surface and translucent skin makes this discoloration much more apparent. And that's why you have those dark under-eye circles staring back at you in the mirror.