This could be misleading but most people that dark circles under their eyes are from lack of sleep.  A person needs about eight hours of sleep each night to be fully rested.  Some have been known to say that an allergy is the cause of dark circles under your eyes.  The bruising causes dark circles under your eyes.  Your blood vessels around your eyes are so tiny that some of the blood is loose in your vessels and that is why dark circles from around your eyes because of any part of your skin, the facial skin is the easiest skin to bruise.  In addition, once the blood breaks down the circles disappear. 

The creams that we buy to make the dark circles disappear actually speed up the process of the blood breaking down and making the dark circles disappear faster.  I went to a beauty class and they told all of there that tanning could cause the dark circles under your eyes darker.  Now with this new knowledge I am still not sure if it is true or not.  I know with creams it helps break down the blood and the blood dissipates faster, making the dark circles under your eyes lighter and eventually disappear.