Some people have the unfortunate luck of dark circles underneath their eyes all the time - regardless of the amount of sleep they get. For other people, the dark circles tend to creep in if they get too tired or didn't get enough rest. What causes these distinctive marks and how can we get rid of them?
At its most basic, dark circles are no more than the color of the blood vessels peeking through the thin layer of skin that's underneath our eyes. The bluish-greenish color that we see is because the skin only lets blue to violet wavelengths pass through.
In some people, their skin produce extra melanin pigment in the skin underneath their eyes. For these people, dark circles are a genetic predisposition that's not likely to resolve with sleep or rest. In others, not having enough sleep or rest causes dilation of the blood vessels, which make these marks more noticeable. Allergies are also notorious for the discoloration of the skin, and for also causing those unattractive eye-bags.
If dilation of the blood vessels worsens the appearance of dark circles, constricting the blood vessels should have the opposite effect. This is why most remedies that involve something cooling, such as cold cumber slices, appear to work. It's not anything special from the cucumber - it's the cold temperature that constricts the veins, which temporarily makes the discoloration less noticeable.