Just like the the caterpillar undergoes a cycle to become a butterfly the hair undergoes a cycle from life to death. The hair goes through a period of growth (Anagen) then regression (Catagen) resting (Telogen) and shedding (exogen). Therefore hair does not continuously grow.
Anagen
Anagen is the hair growth phase. The root of the hair are diving rapidly adding to the hair shaft. During this phase hair grows abour 1-1.25cm every month. Hair will grow for 2 - 7 years. The amout of time the hair folicle stays in this stage is genetically determined.
Catagen
This stage signals the end of the active growth of hair. This phase lasts for about 2-3 weeks. During this stage we may see narrowing of the hairshaft.
Telogen
This stage lasts around four months, it is the resting stage of the hair folicle.
Exogen
At the end of this stage the hair is shed from the head. Fifty to one-hundred hairs are shed daily a normal scalp.
References:
http://www.hairscientists.org
http://en.wikipedia.org/wiki/Hair_follicle
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