The human hair cycle is the cyclical phases of growth of hair. It is comprised of the following stages: anagen, catagen, telogen, and exogen. In certain hair cycles, there is another phase called kenogen.
During anagen, the hair is actively growing and could grow to lengths depending on the type of hair, e.g. vellus hair, terminal hair, or androgenic hair, scalp hair, etc. After the anagen phase, the regression phase called catagen follows. At this stage, the hair follicle shrinks. The hair shaft begins to detach from the dermal papilla and forms club hair, which pertains to a hair shaft characterized by having a club-shaped bulb. The catagen hair is in a resting state. Catagen is also described as the transition phase because it is the shortest phase transitioning from the anagen phase to telogen phase. During telogen (the quiescence phase), the hair follicle is dormant and regresses to half of its size. The club hair is completely formed as well. The hair shaft is eventually shed during exogen. Kenogen may occur in certain hair cycles. It is when the hair follicle is empty for a particular time before the next anagen hair grows.
Each hair fiber goes through these stages. Human hair is not seasonal, which means that different hair fibers go through different phases.
1 Peytavi, U. (2008). Hair growth and disorders. Berlin: Springer. p.10