The growth rate of hair is approximately 0.3 mm to 0.4 mm per day, depending on the person, which is equivalent to some 15 cm a year. Nonetheless, not all hair follicles develop new hair at the same time. Hair growth is produced within a closed cycle. At a given time, each hair can be found at a different stage of this cycle, and there is no synchronisation.
What are the stages of the hair growth cycle?
The hair grows from the follicle or root under the skin. Hair is fed by blood vessels in the base of the follicle which provide hair with the nourishment it requires for growth. Between the first years of growth and the hair dropping out, each hair passes through four stages: anagen, catagen, telogen and exogen.
Over time, the length of the anagen stage decreases. For this reason, the hair may become weaker and thinner after each cycle. This is why it is important to ensure that your diet is rich in specific nutrients to maintain healthy hair growth.
The anagen or growing phase is the first part of the hair development cycle. During the anagen phase, the follicle cells divide rapidly, resulting in new hair growth. In normal conditions, 80% to 90% of hair follicles are in the anagen phase at any given time.
The anagen phase can last between two to seven years. The length of the anagen phase determines the maximum length of the hair. For example, people with very long hair have a very long anagen phase. The eyelashes, eyebrows and body hair have shorter growth phases than the hair on one’s head, which is why they create much shorter hair.
There are many factors that influence the duration of the anagen phase including genetics, nutrition, age and your general health. Stress, hormonal imbalances, medication, poor nutrition, sudden weight loss, aging and even too much styling can reduce the anagen phase. It is important to remember that the hair follicle also has its own biological clock, and when it is out of alignment or under some form of attack, this may interrupt the production of hair. The follicle is nourished by the blood vessels and, when the circulation is obstructed, the hair cannot grow.
After the growth stage, or anagen phase, the hair enters the catagen phase. This short transition phase only lasts for two to three weeks. During the catagen phase the hair stops growing and separates from the blood vessels, that is, from the supply of blood and nutrition.
Hair within the catagen phase is often small and discoloured, as it simply stops growing. For this reason, some people refer to it as “dead hair”.
The telogen phase is the stage that follows the catagen phase and tends to last around 3 months. It takes place when the follicle is at rest before the anagen phase begins once again. Cellular activity within the follicle stops, causing a halt in growth and subsequent shedding when the follicle changes shape. If the percentage of hair in the telogen phase is too high, it may cause telogen effluvium, where a general thinning of hair may be produced. This is why it is important to ensure the scalp is nourished by maintaining a healthy diet, practicing stress control and complementing your diet with nutritional supplements.
The exogen phase is the last stage in the hair growth cycle. In fact, it is merely an extension of the hair’s “rest period”. Eventually, the dead hair is shed, and the new hair begins to grow simultaneously. In normal conditions, one loses around 50 to 100 hairs per day during this phase.
The exogen phase is the reason why we often see hair in the drain of the shower or on our hairbrush. We should only be concerned about this when there is a sudden change and we shed more hair than normal.
After the exogen phase, the follicle returns once more to the anagen phase and the cycle begins again.