All hairs in the body grow in cycles. They grow and then stop for awhile, then shed and grow again. These stages of growth are referred to as anagen, catagen and telogen. The anagen or growth phase is when the hair is most suspectible to the laser During the slow down and dormant phases (catagen and telogen) the laser still has an effect, but it is less. At any time the hairs in a treatment area are in various phases of growth and this is one of the reasons that multiple treatments are required to achieve permanent hair reduction.
With each laser hair removal treatment there is a cumulative damage done to the hair follicle. After being partially damaged, the hair will regrow, but it will be thinner and lighter. With subsequent treatments and a critical amount of damage being reached, the hair will no longer grow. The timing between treatments is chosen to get as many hairs into the active growing stage as possible.