There are different kinds of processes that scalp hair goes through at any given time. It can be growing, resting, shedding or beginning with new hair growth. These will be discussed according to the stages that represent the hair growth cycle.
The first stage is called the anagen or growth phase. It is estimated that eighty-five percent of the hair is undergoing this process at different areas of the scalp. This stage is the longest and can take anywhere between two to seven years. People can have varying anagen stages that allow them to grow hair up to a certain length. Having a shorter growth stage will give them the ability to produce hair at a corresponding length. Asians are said to have one of the longest anagen stages. The growth rate of hair strands during this stage is an estimated one-centimeter at twenty-eight day intervals.
Next is the catagen or transition stage. It is a time where hair growth slowly decelerates and club hair forms. This is evidenced by the shrinkage of the outer root sheath with the hair strand slowly being pushed out after detaching from the dermal papilla. It is the shortest period in the hair growth stage lasting anywhere from one to three weeks. The hair engaged in this stage would be about five percent distributed throughout the scalp.
The telogen or resting phase comes next which accounts for about ten to fifteen percent of scalp hair. This is the time where the club hair formation that began during the catagen stage reaches its completion. Pulling on the hair could potentially release telogen hair strands that would reveal a dry, hard bulb on the opposite end. Left on its own, the club hair will be slowly pushed out of the scalp as new hair begins to develop. The resting scalp hair will go through this process for about two to three months before it moves on the last stage of the hair growth cycle.
The exogen or new hair phase has been regarded as either part of the telogen stage or existing on its own. Whichever it may be, this is the time when twenty five to one hundred fifty hairs are shed on a daily basis. It is also when new hair starts to form and reach the surface of the scalp. This would signal the end of the hair growth cycle and will begin anew with the anagen phase.
These stages will be a regular occurrence as a person ages. There is however, a hair loss condition that can materially effect this cycle because of stress or hormonal changes causing a sudden shift between the anagen to the telogen stage. This is called telogen effluvium or the gradual loss of hair in clumps due to traumatic life events such as accident, major surgery, divorce, childbirth or loss of a loved one. The exact reason why this occurs has yet to be discovered. What can be observed is that hair loss takes place almost three months after the stressful event. It is expected to regrow three months after it has started shedding and should return to its normal state a year after the event that caused it.