There are many reasons why the skin becomes darker, such as the common sun exposure, skin melanosis, and Reye’s melanosis. 1. Sun exposure: ultraviolet light irradiation of the skin can stimulate an increase in the activity of melanin in the skin, which leads to darkening of the skin as well as wrinkles and aging and other phenomena. 2. Melanosis: a group of skin diseases clinically characterized by skin pigmentation, the cause of which is very complex, including genetic, physical and nutritional deficiencies, exposure to certain chemical substances, etc., can lead to patients with varying degrees of local skin pigmentation spots, but also accompanied by itching, burning, and fatigue and other discomforts. 3. Riehl’s disease: belongs to a kind of pigmentation disease mainly occurs in the face, common in middle-aged women, the cause of the disease has not been clarified, with the long-term use of cosmetics containing photosensitive substances, or the body lack of vitamins and long-term exposure to the sun and other factors caused by photosensitive dermatitis. In addition, we cannot rule out the possibility of other causes of skin darkening. It is recommended that patients go to the hospital in time to find out the cause of the disease, and actively treated under the guidance of the doctor.