Wikipedia Answer
Causes
[edit] Heredity
Like varicose veins, dark circles under the eyes are usually an inherited trait. If you have dark circles, there is a good chance that others in your family also have them. The skin under the eye is very thin. When blood passes through the large veins close to the surface of the skin it can produce a bluish tint. The more transparent your skin — also an inherited trait — the darker the circles appear.
[edit] Bone structure
If you have deep-set eyes, shadowing contributes to the dark color under the eyes.
[edit] Pigmentation
Genetic hyper-pigmentation is a common form of dark circles. Even in darker skinned people, exposure to sunlight, especially during the summer months, can cause a higher-than-normal level of skin pigmentation (melanin) under the eyes. People get sun tans because exposure to the sun increases the natural pigmentation of the skin and draws that pigmentation to the surface. The same principle applies to the skin under the eyes.
[edit] Allergies, asthma, and eczema
Any condition that causes the eyes to itch can contribute to darker circles due to rubbing or scratching the skin around them. Hay fever sufferers particularly will notice under-eye "smudges" during the height of the allergy season. Some food allergies can also cause the area under the eyes to appear darker.
[edit] Medications
Any medications that cause blood vessels to dilate can cause circles under the eyes to darken. Because the skin under the eyes is very delicate, any increased blood flow shows through the skin.
[edit] Lack of nutrition
The lack of nutrients in the diet, or the lack of a balanced diet, can contribute to the discoloration of the area under the eyes. It is believed that lack of mineral iron can cause dark circles as well. Iron deficiency is the most common type of anemia and this condition is a sign that not enough oxygen is getting to the body tissues.
[edit] Fatigue
A lack of sleep or excessive tiredness can cause paleness of the skin, which again allows the blood underneath the skin to become more visible and appear more blue or darker.
[edit] Pregnancy and menstruation
The skin can also become more pale during pregnancy and menstruation (due to lack of iron), which again allow the underlying veins under the eyes to become more visible.
[edit] Age
If you have a propensity to have dark circles under you eyes, as you grow older, they are likely to become more noticeable and permanent. Excess folds of skin under the eyes will also make dark circles more pronounced.
[edit] Blood Circulation
Another cause may be bad blood circulation. Cardio exercise increases blood circulation and may help the dark circles to disappear.
[edit] Kidney or Liver problems
The area directly located under the eyes are closely related to kidneys and liver, so the appearance of dark circles may indicate problems or weakness in the kidney or liver activity.
[edit] Dehydration
Dark eye circles could be a sign of excessive loss of water from the body