The hair of the scalp grows at a rate of 1.25 cm per month. And this means approximately 15 cm in a year. Hair grows in cycles. A hair follicle goes through several phases. When these phases are taken together, we then speak of a hair cycle. One hair cycle may last up to seven years.

During the growth phase (anagen phase, length of duration is around 3 to 7 years), the cells in the hair bulb continuously divide resulting in hair growth. A short transitional phase follows (catagen phase, lasting for around 2 to 5 weeks), which is initiated by a sudden stop in the division of cells in the hair bulb. The hair follicle shrinks substantially, the hair shaft grows no more since it no longer receives a supply of horn cells. However, the shaft remains stuck to the shrunken hair follicle.  The rest phase follows (telogen phase, lasting 3 to 5 months). In this phase, it is observed that the shrunken follicle rests.

       

Anagen                                     Katagen                                            Telogen

 

However, nobody can adequately explain what exactly happens at the end of the telogen phase. As if touched by the magic wand of a wizard, the shrunken hair follicle suddenly "wakes up" and a cell division in the hair bulb begins anew.  The new hair pushes the old one upwards until it falls. Oftentimes, the old hair already falls during the telogen phase, in view of the loosening of connective tissues.

Since 85 % of the hair are in anagen phase (1 % in catagen phase and 14 % in telogen phase) in a given period of time, nothing actually changes in the total number of visible hair.