Physiology of the hair growth cycle

During a person’s lifetime, each hair follicle goes through many cycles of hair growth. Hair growth cycle of hair follicles on the scalp describes the 3 stages of hair growth: anagen, catagen and telogen. At any one time, most scalp hairs are in anagen (ie the growth phase of the cycle), which involves maintaining scalp hair volume and enabling hair to grow longer. Hairs in telogen (ie the shedding phase of the cycle) are often shed during hair washing and brushing. Catagen is the transition phase between anagen and telogen.

Hair loss disorders are usually due to the hair growth cycle being interrupted at different stages.

Table 1. Hair growth cycle of hair follicles on the scalp
Phase of hair cycle	Description	Duration	Relative proportion of hairs in this phase at one time
anagen	active hair growth [NB1]	2 to 6 years	85 to 90%
catagen	transition	1 to 2 weeks	fewer than 1%
telogen	resting (also known as the shedding phase) [NB2] telogen hairs have a white bulb	20 weeks	10 to 15%
Note: NB1: Hair growth of approximately 1 cm per month can occur. NB2: Hairs are often shed during hair washing and brushing; it is normal to shed up to 100 hairs daily.