Alopecia Areata Physiology
Body hair grows in cycles, which are different in various areas of the body. Scalp hair has a growth phase that lasts for 2 to 6 years. During this time, called the anagen phase, individual hairs will grow about one-half inch per month. About 80-90 percent of scalp hair is in this part of the growth cycle.
The anagen phase is followed by a short period of time called the catagen phase. This lasts a few weeks while a hair follicle stops growing. The hair follicle then enters a resting or telogen phase. About one in ten hair follicles are in the resting phase at any one time. This period lasts for about five months, and the hair falls out. After a period of time, a new anagen phase will begin.
- Lenane P, Pope E, Krafchik B. Congenital alopecia areata. J Am Acad Dermatol. 2005 Feb;52(2 Suppl 1):8-11. 
- Rainforth SI. Historical diagnostic and treatment. Alopecia areata. Skinmed. 2005 Jan-Feb;4(1):5. 
- Sladden MJ, MacDonald Hull SP, Wood ML, Hutchinson PE, Messenger AG. Alopecia areata: the need for guidelines and evidence-based dermatology. Br J Dermatol. 2005 May;152(5):1086-7.