Human hair contains a pigment called melanin, which is the source of your hair color. When you lose most of your melanin, your hair turns gray; when you lose all of it, your hair turns white. You will inevitably go gray--and eventually white--as you age, but there are other factors that may cause your hair to turn white prematurely.
Age
Most people lose the coloring in their hair as they get older. When you're young, your body constantly adds pigment to new hair cells. But as you age, the cells mature and stop producing new melanin. Most men notice their first gray hairs around age 30; they begin showing up in women around age 35.
Genetics
The age at which you go gray is heavily influenced by heredity. If your parents and other older relatives started graying at a young age, chances are you will, too. Similarly, genetics has a hand in determining whether a man will go white, go bald or both.
Disease
Some medical conditions have been linked to premature gray and white hair. These include thyroid problems, vitiligo and anemia. Sometimes, not getting enough vitamin B12 in your diet will lead to earlier graying. Some cancer patients have experienced loss of hair pigment during chemotherapy treatments.
Cell Stress
A study by Tokyo Medical and Dental University found that "cell stress," or DNA-damaging events, can also cause hair to lose its pigment. Some examples include exposure to chemicals, ultraviolet light and radiation. Cellular stress is not to be confused with physical stress, which some people believe causes gray or white hair; however, the evidence to support that theory is inconclusive. In rare cases, an autoimmune condition called alopecia areata sometimes causes dark hairs to fall out quickly, leaving the appearance of hair "turning white overnight."
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