There are approximately 65-90 hair follicles per square inch of the human body. That's a lot of hair!! These hairs are all in different stages of hair growth, known as:
Anagen - Actively growing hair stage.
Catagen - Shrinking stage, still receiving nourishment.
Telogen - Resting stage, before hair is shed.
New hairs are always being generated, and may be growing in the follicle even before the visible hair has been shed. The anagen stage is the optimum treatment stage, but hairs in any stage can be treated.