Anagen, categen and telogen
Hair growth cycle is divided into 3 phases. Anagen, catagen and telogen.
Recent advances have added more stages/phases but for ease of understanding, I am limiting this discussion to easy to understand simple stages.
The active growth phase is called anagen. During anagen, cells in the root/hair buld divide mitotically to add to the hair shaft.
During catagen, the hair bulb ceases to divide and starts involuting and regressing. Catagen phase starts at the beginning of involution and ceases when the involuting bulb reached the level of the bulge.
Telogen is the resting phase of the hair. The hair ceases all activity and stays as a club hair. It may shed at any time from the beginning of the telogen to the time new anagen phase commences.
The event at which the telogen hair sheds is called exogen.
Exogen need not be at the time of the start of the next anagen. Effects like traction, trauma may bring it earlier on in telogen.
The various phases of the hair growth cycle differ in different hair (scalp, nape, body, beard hair etc. have different duration of the various phases).
For a present, we concentrate on the scalp hair.
Anagen lasts for anywhere between 2 to 6 years. Catagen for 2 to 4 weeks and telogen for 3 to 5 months.
As a thumb rule, we can say A:C:T is 3yrs:3weeks:3months.
In scalp hair approximately 85% hair are in anagen, 12 to 14% hair are in telogen and 2 to 4% hair in catagen.