It is melanin, produced by melanocytes, that causes the skin to darken which is found on the skin's epidermis. Overproduction or underproduction of melanin results to skin discoloration.
That is the reason why too much sun exposure results to skin darkening - when melanocytes produce more melanin (hyperpigmentation) than they should, they turn into freckles or moles.
For additional information about skin care, please check the Sources and related links section.
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You have three layers of skin; the epidermis, the dermis, and the hypodermis; as you may already know. Your epidermis regenerates itself completely ever 3-5 weeks; which is very fortunate; because there are some pretty important structures in there. Melanocytes, one of those important structures, are special cells that produce a pigment called melanin. Melanin gives your skin its color; so the more melanin you have, the darker your skin. People around the world have different amounts of melanin in their skin. As an example, people from Africa have lots of melanin in their skin; while people in Scandinavia don't have much at all. Well anyway, melanocytes produce more pigment (melanin) when they are exposed to sunlight. That explains why you get a tan when you have been out in the sun all day. I really do hope this helps!
Many factors can cause skin darkening e.g. sun light , MSH , UV light ,addison's disease etc.