Human hairs do not grow at the same speed and intensity throughout one's life. Growth takes place in phases, in constantly repeating cycles, which are not always of the same intensity and regularity.
A single hair cycle consists of 3 phases:
1. The anagen or active growth phase
The first phase is also known as the active phase, as it is the only phase during which the hair root cells actually produce hair. In this phase the hair bulb remains deep in the skin, where it remains until the resting phase. Some 85 % of all hair on the head will be in the active phase at any one time. The anagen phase can last 2 - 6 years. Hair grows on average 10 cm per year, and any single hair can grow to be over one metre long. A scalp contains about 1 million hair follicles / hair roots, though generally speaking only 100,000 - 150,000 hairs are actually visible. This gives us an indication of how many hair follicles are not active.
2. The catagen or transition phase
At the end of the growth phase, hair moves into a short transition phase lasting 1 - 2 weeks. In this period the hair follicles get ready for the resting period. At any one time, some 2 % of all hairs will be in this transition period. During this phase, hair growth stops, with hair follicles shrinking to about 1/6 of their normal length.
3. The telogen or resting phase
Coming at the end of the transition phase, the resting phase generally lasts 5 - 6 weeks. During this period a hair does not grow at all. It nevertheless remains connected to the follicle, while the dermal papilla sinks down to the bottom, going into a resting phase. Some 13 % of all hair will be in the resting phase at any one time, though this percentage can vary between 4 % and 24 %. At the end of this phase the older hair falls out, with a new hair finding its way through the skin to the surface, where it will remain for the next few years. The hair bulb returns to its usual place and the hair re-continues its healthy growth. In the course of a person's life this cycle will be repeated 20 times on average.