In the section above, we explained how hair growth takes place in the hair follicle. This growth, however, is not continuous and can be divided into two distinct phases. We will refer to these phases as the growth phase or anagen and the resting phase (technically the resting phase can be further divided into two different sub-phases, catagen and telogen, but we will not differentiate these).
Hair growth cycles. Retrieved from Open Learning Initiative, Anatomy & Physiology (Open +Free).License: Creative Commons Attribution 3.0 Unported License. Link: https://books.byui.edu/-owBA
During the growth phase, as the name implies, the hair is actively growing by the process explained above. The length of this phase varies from person to person as well as in hair of different parts of the body. Estimates for the length of the growth phase for hairs of the scalp range from 2 to 10 years while the growth phase for the eyelashes may be as short as 30 days. Once the hair stops growing it enters the resting phase. Again, the length of this phase varies, probably lasting about 4-5 months in the hair of the scalp and 3 months in the hair of the eyelashes. At the end of the resting phase a new growth phase begins. As it does the old hair shaft is pushed out of the follicle as the new hair begins to grow. It is normal to shed about 50-100 hairs from the head per day as new growth phases begin. It should be noted that each follicle acts independently of the others so there is no synchronization of the phases. Also, at any given time it is estimated that around 90% of the hair follicles of the head are in the growth phase. The average rate of growth during the growth phase is about 0.3 mm per day, but again there are differences between individuals and between hair at different parts of the body. Based on what has been explained here, how would you explain the fact that some people can grow hair down to their knees while others have trouble getting it past their shoulders?
Here are a few other interesting facts about hair:
- Our hair grows fastest between ages 15 and 30.
- Hair growth is slowest in infants and in the elderly.
- Hair grows faster in summer than in winter.
- The month where hair is shed from the follicle the most is November (at least where we live in the northern hemisphere)
- Female hair grows faster than males.
- Blondes have about 10% more hair follicles than brunettes and redheads have about 10% fewer hair follicles than brunettes.