4 Answers

Because light bags would blind you.

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- Allergies

- Atopic dermatitis (eczema)

- Heredity — dark under-eye circles can run in families

- Lifestyle factors, such as physical or emotional stress, smoking, or chronic alcohol use can take a toll on your appearance

- Nasal congestion (which can dilate and darken the veins that drain from your eyes to your nose)

- Pigmentation irregularities — these are a particular concern for people of color, especially blacks and Asians

- Rubbing or scratching your eyes

- Sun exposure, which prompts your body to produce more melanin, the pigment that gives skin its color

- Thinning skin and loss of fat and collagen — common as you age can make the reddish-blue blood vessels under your eyes more obvious

http://www.thebeautyinsiders.com/remedies-for-under-eye-dark-circles.html

Maybe you do not get enough sleep. Sometimes dark circles under the eyes can be kidney associated but only sometimes and depending on your age. Also depends what colour your skin is i.e. pigmentation.

There called eyelid bags some time being over tired. Or a herniation of fat which lies beneath the eye lid it can be remove or there is a creams. Check with your doctor for details.