Allergic shiners occur when congestion in your sinuses leads to congestion in the small veins under your eyes.
Darkened skin patches occur as a consequence of heightened melanin production, which can be triggered by factors ranging from sun exposure to genetic predisposition.
Anemia or an iron deficiency can result in dark eye circles as not enough oxygen reaches the body's tissues.
Prolonged sun exposure or injury may cause dark circles to form under your eyes.