Hair growth is not a continuous process, it has 3 stages -

ANAGEN PHASE:
The first phase is the growing stage. Hair grows at about 1 cm each month, and this phase lasts between 2 and 6 years.

CATAGEN PHASE:
As this phase is the partial degeneration stage and begins when the bulb detaches from the blood supply and the hair shaft is pushed up. This phase lasts between 10 and 14 days.

TELOGEN PHASE:
This is followed by a resting stage, during which there is no growth. This phase lasts between 3 to 5 months and there is an involution of the hair follicles during the period.

At the end of the resting phase, the hair is shed, and the follicle starts to grow a new one.

At any moment, about 90% of the hair follicles of the scalp are growing hairs in the first phase; only about 10% are in the resting phase.

If a follicle is destroyed for any reason, no new hair will grow from it.

Telogen hairs all have club-shaped "roots" typical of resting hairs and may be easily identified under the microscope. After shampooing, the bulk of existing loose hairs has often been shed and loose hairs may be taken from the scalp as a biopsy to be examined under the microscope. In this way, the condition of the hair follicles, the tissues that produce the hair, may be determined.