The hair growth cycle consists of three principal phases: the anagen (growth) phase, the catagen (transition) phase, and the telogen (resting) phase. During anagen, follicles are active and hair strands grow steadily—on the scalp this phase may last several years. The catagen phase is very brief, lasting only a few weeks; it marks the end of active growth as the follicle detaches from its blood supply and begins to shrink.  In the telogen phase, growth ceases while the follicle rests and prepares for renewal; at the end of this phase, hair may be shed and the cycle begins again.

The duration of each phase varies greatly depending on follicle location, genetics, and hormone sensitivity. For example, body hair (including chin hair) has a much shorter anagen phase compared with scalp hair, which explains why it remains shorter and slower-growing. Hormones, particularly androgens, play a central role in stimulating hair follicle activity in these phases; a higher level of androgens or greater sensitivity of the follicles to them can lead to thicker, more visible hair growth in areas like the chin in both men and women.

Understanding this cyclic nature of hair growth is key. Because not all follicles are in the same phase simultaneously, hair length and shedding remain relatively stable. However, disruptions or hormonal changes can alter the timing or duration of these phases, affecting hair volume, thickness or rate of shedding.