Dark circles under the eyes can be indicative of kidney or stomach complaints. This may be because of serious health issues -- notably problems with your adrenal glands -- that should be treated by a physician but they can be for less drastic reasons. Skipping meals, drinking too much caffeine and even having fruit juice or fruits with meals may be the culprit.
If you have other symptoms of kidney problems (burning during urination, lower back pain, etc) you should seek medical advice immediately. So too if you are experiencing severe stomach problems.
If you do not appear to be suffering from any serious condition but you do drink a great deal of caffeine or fruit juice, try to reduce your intake. You should eat regularly and ensure that your diet is balanced and rich in vitamins and minerals.