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is multi factorial,there is no one reason to it..and thats the reason even the treatment has to be a combination of few things .Common reasons for dark circles are iron deficiency
,lack of sleep,stress,thin skin around the eyes(fine lines),less water intake,any chronic illness,excess sun exposure,familial etc.
Various treatments available are-chemical peels
cream,tretinoin cream,kojic acid,vit c creams etc.Latest would be use of small tiny injections which would help filling the sunken area around the eyes.Also one can try lasers which are specific for pigmentation like Q-switched Nd:yag laser (1064nm).
Along with this improve your sleeping habits ,drink plenty of water ,use a good sunscreen daily and follow healthy life style .
You should understand that you may not get 100% result with any of the methods ,but improvement is what you should look at and try maintain what results you get.
Consult a dermatologist
who can analyse all the causes and has latest facilities for the same.