The anagen phase is known as the growth phase. It begins in the papilla and can last up to eight years.
The span at which the hair remains in this stage of growth is determined by genetics.
The longer the hair stays in the anagen phase, the faster and longer it will grow. During this phase, the cells in the papilla divide to produce new hair fibbers, and the follicle buries itself into the dermal layer of the skin to nourish the strand.
About 85% of the hairs on one's head are in the anagen phase at a given time.
Signals sent out by the body determine when the anagen phase ends and the catagen phase begins.
The catagen phase, also known as the transitional phase, allows the follicle to, in a sense, renew it.
During this time, which lasts about two weeks, the hair follicle shrinks due to disintegration and the papilla detaches and "rests", cutting the hair strand off from its nourishing blood supply.
Ultimately, the follicle is 1/6 its original length, causing the hair shaft to be pushed upward While hair is not growing during this phase, the length of the terminal fibbers increase when the follicle pushes them upward.
During the telogen, or resting, phase the hair and follicle remain dormant anywhere from 1–4 months.
Ten to fifteen percent of the hairs on one's head are in this phase of growth in any given time.
The anagen phase begins again once the telogen phase is complete. The preceding hair strand is pushed up and out by the new, growing strand. This causes a normal hair loss commonly known as shedding