There is a genetic predisposition of each hair follicle to follow a particular pattern of growth and rest throughout a persons life. Hair follicles are programmed to stop producing hair and spend more time in the resting stage with the progression of age.

The growth cycle of a hair consists of 3 stages:

1. ANAGEN (GROWING) STAGE

This is the growth phase of the hair. The hair grows about 0.4mm per day. The anagen stage for the hair follicles in the scalp typically lasts about 3 to 5 years.

2. CATAGEN (TRANSITIONAL) STAGE

This transition period of a hair follicle from growth to rest is called the catagen stage When the  hair follicles prepare themselves for the resting phase. This stage of the hair growth cycle usually lasts about 1 to 2 weeks and about 3% of all hair are in this phase at any one time. During the catagen phase the deeper portions of the hair follicles start to collapse.

3. TELOGEN (RESTING or SHEDDING) STAGE

This is the resting period of a hair follicle where the hair is essentially shed. It accounts for 6-8% of all hair. It is usually 3 to 4 months in length and at the end of this period older hairs that have finished their life will fall out and newer hairs will begin to grow.

The growing (anagen) phase constitutes about 90% (1000 days or more) of the growth cycle of a hair follicle, while intermediate (catagen phase; 10 days) and shedding (telogen phase; 100 days) phases constitute only 10% of it.