Dark circles under the eyes are a common complaint of both men and women, although they can occasionally be seen in children. As people age, the skin becomes thinner and collagen is lost, sometimes enhancing the appearance of blood vessels beneath the eyes and making the area appear darker. Dark circles under the eyes are not necessarily a sign of tiredness, but stress and fatigue seem to worsen the facial appearance of many people, including their tendency to develop dark circles.
Conditions like fluid imbalance or local swelling that lead to puffy or swollen eyelids can cause shadows that make the area under the eyes appear darker. Trauma (a "black eye") can cause darkening of the entire eye area. Some people notice that dark circles under the eyes tend to run in families, as well.
In most cases, dark circles under the eyes are not a sign of a serious medical condition.