1) Lack of sleep: the skin becomes paler due to this reason. Lack of sleep also reduces circulation. You need at least 7-9 hours sleep per night. You should reduce your stress; it helps you to sleep more. Alcohol and other drugs can affect the quality of your sleep so abstain from them.
2) Vitamin deficiencies: A big reason of dark circles is vitamin deficiencies particularly vitamin k and B12.you should eat a healthy and balanced diet that contain large amount of vitamins. You can also take a daily vitamin supplement if necessary.
3) Allergy: Allergies are also a common cause of dark circles under the eyes especially Gluten intolerance .it is an allergy to wheat flour.
4) Face structure: In some people the bone structure of face and add shadows under eyes, making them look even darker then they are.
5) Hyper-pigmentation: it is also a big reason of black-circled eyes; it is a condition that is passed on due to heredity.
6) Vascular inflammation: it caused by enlarged blood vessels beneath the skin under eyes. Allergies can cause the inflammation.
7) Excess salt intake: the body retains water in unusual places by eating too much salt, which can result as black circles and puffiness under the eyes. It also disturbs the blood circulation, in result; the blood vessels under the skin appear bluer.
8) Lifestyle factors: Poor nutrition, using medicines that contain alcohol, smoking, and lack of exercise can also cause the eye discoloration.