Hyperpigmentation is darkened areas of the skin due to sun damage or inflammation. This is caused by excessive production of melanin (the pigment that produces darker color in the skin).
One reason for hyperpigmentation is sun exposure. UV rays stimulate melanocytes that produce melanin to protect the skin from damage. In addition, melanocyte regulation becomes weak with age–this is why elder individuals have sun spots and other skin pigmentations. Other factors, such as chemotherapy drugs, pregnancy and endocrine diseases can increase the production of melanin as well. Other skin conditions can cause hyperpigmentation, too.