The area under the eyes can appear darker due to constricted blood vessels causing hyperpigmentation, or from thinning of the skin around the eyes.
All causes of dark circles under the eyes include:
-lack of sleep or poor sleeping habits
-allergies, including hay fever
-hyperpigmentation, which happens when the body produces more melanin
-reduced levels of fatty tissue around the eyes
-thinning skin under the eyes
-anemia from iron deficiency
-overexposure to sunlight
-frequent rubbing of the eyes
-aging
-smoking
-inherited genes
-thyroid conditions
-dehydration
-dermatitis