Having dark circles under the eyes is not a sign of illness, but it can certainly make you look sick, tired or older than your years. Sometimes genetics plays a role, making it difficult or impossible to ever get rid of them. However, knowing what causes them can help you be proactive in minimizing their appearance.
Heredity plays a big part in how apparent dark circles under your eyes are, but there are other causes, as well, including allergies and nasal congestion---both of which contribute to congestion of blood under the eyes.
One misconception is that lack of sleep causes dark circles, but staying up past your bedtime may give you puffy eyes the next morning, which then cast a shadow and give the appearance of dark circles. Pale skin caused by lack of sleep also exaggerates dark circles.
The skin underneath the eyes is the thinnest on the entire body and easily exposes the capillaries---the body's smallest blood vessels---resulting in blue-purple shadows. These shadows are actually the effect of broken or leaking capillaries (a fairly normal occurrence, much like getting a bruise). Fluid from these leaky capillaries, as well as hemoglobin---the protein in red blood cells that carries oxygen---gradually collects below the eyes and forms what look like dark circles.
An obvious effect of dehydration is dark circles under the eyes. A lack of water, as well as smoking, alcohol and caffeine, can deprive the skin of moisture and prevent vitamins from being absorbed by the body, particularly vitamin C, needed to restore skin cells. Smoking also slows down the blood flow and causes blood vessels to leak. On the flip side, water retention can cause swelling under the eyes, which emphasizes dark shadows.
Overexposure to sun can be harmful to the skin, especially for the delicate skin under the eye. Sunlight increases the natural pigmentation of the skin, which makes dark circles you have appear even darker.
Our skin becomes thinner as we age due to a loss of fat and collagen, which is especially noticeable in the tissue around the eyes. The thinner skin makes the bluish vessels under the eyes more prominent.
Apply a cold compress, such as two chilled teaspoons, frozen peas, slices of refrigerated cucumber or cooled teabags, to the eyes to diminish under-eye "bags" so that dark shadows appear less prominent. Get adequate sleep so that a pale face and hollow eyes do not exacerbate any dark shadows. Elevating your head while sleeping will prevent fluid from pooling in the face and diminish puffiness. Drink plenty of water (eight 8-ounce glasses per day) to keep your skin hydrated and healthy. Quit smoking and ease up on the alcohol and caffeine. See your doctor about any allergies you have or think you have and get them treated. If you use concealer makeup under the eyes, choose one with peach tones to counteract the blue-tinted shadows. Use a moisturizer with Vitamins K and C to replenish the skin's collagen.