The Anagen phase is the growing phase or the 'on' phase, which lasts for an average of approximately 1,000 days in the human scalp, but can range from two to six years. During the Anagen phase, scalp hair cells grow rapidly at an average rate of one-half to one inch per month. As hair grows, the cells of the hair move closer to the skin surface, being replaced along the way by new, rapidly reproducing cells below. The hair shaft grows in diameter and the hair reaches its maximum length.