The human hair follows a unique growth cycle. Unlike other mammals which grow or shed hair depending on the season, human hair growth is considered random but follows three distinct phases occurring simultaneously.
Stage 1: Anagen Phase
This is the active phase of hair growth. During anagen, cells in the root of the hair are dividing rapidly, forming new hair and pushing out old hair that has stopped growing. Normally, 90% of the hair follicles are in the anagen phase at any given time, with an average growth rate of 1 centimetre every 28 days. The scalp hair usually remains in the growing phase between 2 to 6 years compared to hair in other parts of the body, such as the eyebrows, legs, and eyelashes, which have an anagen phase of about 30 to 45 days only.
Stage 2: Catagen Phase
The catagen phase is also known as the transitional stage in hair growth that occurs at the end of the anagen phase. During this phase, the active growth of the hair stops and a club hair is formed. A club hair is formed when the hair follicle shrinks and detaches itself from the papilla, described as a knoblike indentation at the base of the hair follicle, which contains the blood supply necessary for the hair’s nourishment. About 3% of all scalp hair is in the catagen phase at any given time and lasts for about 2 to 3 weeks while a club hair is formed.
Stage 3: Telogen Phase
This is the last stage in the hair growth cycle and also referred to as the resting phase. Lasting for about 100 days, around 10% to 15% of scalp hair is in this stage at any time. During telogen, the hair follicle completely stops growing and the club hair is fully formed. Towards the end of this stage, hairs that have completed the cycle will fall off naturally and new hair will begin to grow. Around 25-100 hairs may fall out daily as part of the normal shedding process.
The rate and period of hair growth varies from one person to the next depending on genetics, metabolism, diet and nutrition, age, race, hormones and other factors but what is common to all is the three stages of distinct hair growth that every hair follicle goes through.