I'm dealing with this too. I've been researching it. My eyes are darker and puffier lately. Puffiness and darkness each make the other look worse. Puffy eyes cast a shadow and make them look darker. Dark eyes emphasize the puffiness.
For dark eyes, it can be from from allergies. There's a term called "allergic shiners". Other terms are "periorbital hyperpigmentation", "periorbital darkening". If it's from bruising then it's called "periorbital ecchymosis" or "raccoon eyes". Dark circles can be caused by your eyes being directly allergic, itchy, inflamed, rubbing them, etc. It can also be caused by long standing sinus congestion. Either way you should see an allergist. There are antihistamines, and things like Nasonex which may help. But, the ultimate would be to get allergy shots and cure yourself of the allergy after several years. You can get dust mite covers for your pillows and mattress. It's a common allergen that can mostly be avoided. There are other things you can to to help minimize your allergies, like air filters and what not.
If dark eyes are not from allergies. It can be from alcohol or lack of sleep, which is basically stress. Though I have read that lack of sleep does not cause it, instead it causes your skin to look pale which emphasizes your already dark eyes. I'm not sure I believe that.
The technical term for puffy eyes is: periorbital edema. Or, it could be: fatty orbital herniation. If the puffiness is just below the eye on the cheekbone, then it's a festoon is a malar bag. Puffy eyes can be from dehydration. Drink more water. Though I have also read that too many liquids before bed can make it worse. Eat less sodium. Eat more potassium. Another thing you can do is raise the head of your bed 3-6 inches. Sleep on your back. It can be caused by thyroid disease, and people with thyroid problems can have a very bad time with puffy eyes.http://www.mayoclinic.com/health/dar...r-eyes/AN00423http://saveyourface.com/whatsnew/200...ent-deformity/http://drseckel.com/puffiness-of-the...-and-eye-bags/http://pmid.us/17760701
(Your library can get you the full study)