Periorbital dark circles
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In most cases, the dark circles under the eyes are blood vessels that can be seen through the skin. The skin around the eyelids (periorbital skin) is the thinnest skin in the body (around 0.5 mm thick compared with 2mm in other areas). Like varicose veins, dark circles under the eyes are usually an inherited trait. When blood passes through the large veins close to the surface of the skin, it can produce a bluish tint. The more transparent the skin—also an inherited trait—the darker the circles appear. In people with a deep-set bone structure, shadowing can also contribute to the dark color under the eyes.
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 in particular 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.
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.
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 iron deficiency 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.
A lack of sleep can cause paleness of the skin, allowing the blood underneath the skin to become more visible and appear bluer or darker.
Dark circles under eyes can be symptom of liver disease.
Dark circles are likely to become more noticeable and permanent with age. This is because as people get older, their skin loses collagen, becoming thinner and more translucent. Circles may also gradually begin to appear darker in one eye than the other as a result of some habitual facial expressions, such as an uneven smile.
Periorbital hyperpigmentation is the official name for when there is more melanin produced around the eyes than is usual, giving them a darker color.