Composed of 120,000 to 150,000 individual hairs, our hair renews itself spontaneously throughout our life. Each hair independently grows, dies, falls out and grows again, according to an unchanging cycle lasting approximately 3 years for men, and 4 to 7 years for women.
During our lifetime, our hair grows, falls out, and grows again. But over the years, the hair follicles are altered by various factors: age, hormonal changes, oxidative stress, diet, pollution, etc. The follicles then produce ever finer hair until their activity stops completely, leading to a loss of hair density.
- The three phases in the life of a hair
Hair growth is not continuous. It occurs in three successive phases:
– The anagen phase, which lasts 3 to 7 years depending on the sex and the individual, and during which the hair grows regularly by approximately 0.3 mm per day (or 1 cm per month). At this stage, the hair root is buried deep in the dermis. It swells and fills the hair follicle. New keratinocytes are produced in the matrix and push the oldest ones out, thus lengthening the hair.
– The catagen phase, which lasts 3 to 4 weeks, and during which the hair bulb’s keratinocytes degenerate and the hair stops growing. The bulb loses its volume and slowly moves up towards the surface of the scalp, thereby breaking off communication with the dermal papilla, resulting in its death.
– The telogen phase, which lasts 3 to 4 months: the hair is shed naturally as a consequence of rubbing, brushing, and the pressure of a new root forming beneath the scalp. This is followed by a rest period during which nothing happens, before a new cycle begins.
- Asynchronous growth
In spite of these different phases all of our hairs go through, our hair density remains visibly constant (except where a disorder is present). This is because each hair’s cycle is independent of the other hairs’: they aren’t all shed at once.
At any one time, approximately 90% of our hair is in the anagen phase. As such, the hundred or so hairs we lose each day go unnoticed.
When certain disorders are present, the proportion of hairs in the telogen phase can increase by up to 30% and in these circumstances the rapid loss of hair density is noticeable.
- A variable cycle
The hair cycle varies from one individual to the next, as it is influenced by many factors:
– Age: during childhood, the percentage of hairs in the anagen phase is at its peak. It then drops slowly from puberty onwards.
– Ethnic origin: Asians, Blacks and Caucasians don’t have the same hair cycle. The anagen phase, for example, is longer in people of Asian origin.
– Sex: the anagen phase lasts longer in women than in men, which is why women’s hair grows longer.
– Male hormones (androgens): they have a strong influence on the hair cycle, which they can speed up excessively. The hair follicle then reaches its production capacity much quicker and the hair dies completely.
– The season: there are more hairs in the telogen phase in spring and in autumn. It is therefore normal to notice more hair being shed during these periods.
– Diet: certain deficiencies can shorten or even stop the growth phase of hair. Protein, vitamins, minerals and essential fatty acids are essential to keeping hair healthy.