Normal skin is around 2 millimeters thick, but eyelid skin is only 0.5 millimeters thick. Bags under the eyes, formed of fatty tissue and fluid, form early in some families. Shadows from overhead lighting may cause the appearance of circles, or thin skin on the lower eyelid may let the dark-red or purple inside of the lid show through. If fluid is retained in the body, the bags can get even bigger. Some conditions that cause fluid retention include heart, thyroid, kidney and liver problems, high blood pressure and premenstrual bloating.
Some medications can also cause fluid problems.
Then, as the body ages, the skin around the eyes may thin and sag, causing bagginess. Fatigue may make muscles relax or lose their tone, with the same results, but fatigue is probably not as big a cause as the other three. To avoid dark circles, cut salt intake, to fight fluid retention, and try sleeping with extra pillows, to let gravity fight the pooling of liquid. Sometimes the solution to circles and bags may be as simple as stopping the use of an irritating cosmetic, like mascara, which may contain a dye to which some people are allergic. In some people, puffiness results from a protrusion of excess fat around the eyes.