Hairs grow from hair follicles, which are miniature organs in the skin.
Each individual hair follicle grows a single hair in a cycle consisting of a long period of growth followed by a relatively short period of rest. During the rest period the hair is still attached to the hair follicle, but it is not growing. After each rest period, the hair is shed, and a new hair begins to grow, and the growth cycle starts again. The life of a given hair is measured by adding the growth period plus the rest period. Each hair follicle has a genetic predisposition to follow a particular pattern of growth and rest.