HAIR GROWTH CYCLE
Unlike other mammals, human hair growth and loss is random and not seasonal or cyclic. At any given time, a random number of hairs will be in various stages of growth and shedding. There are three stages of hair growth: catagen, telogen, and anagen.
The catagen phase is a transitional stage that lasts for about 2-3weeks and about 1% of all hairs are in this phase at any time. During this time growth stops and the outer root sheath shrinks and attaches to the root of the hair. This is the formation of what is known as a club hair.
Telogen is the resting phase and accounts for 10-15% of all hairs. This phase lasts for about 3 to 4 months. During this phase the hair follicle is at rest and the club hair is completely formed. These are the hairs you lose when shampooing or combing your hair, about 25-100 Telogen hairs are shed normally each day. At the end of this phase, the hair follicle re-enters the Anagen phase.
Anagen is the active phase of the hair. The cells in the root of the hair are dividing rapidly. If the old hair has not already been shed during the Telogen phase, the new hair will push the club hair up the follicle and eventually out. During this phase, the hair grows about 1 cm every 28 days. Scalp hair stays in this active phase of growth for 3-10 years. Some people have difficulty growing their hair beyond a certain length because they have a short active phase of growth. At any one time, about 85% of the hair on a person's scalp is at this growing (anagen) phase. However as people age, their rate of hair growth slows.