In his new book Hair: A Human History, Kurt Stenn explains a myriad of facets about hair growth and it’s distribution and development in humans, but of great interest to me was his explanation as to why hair has four phases of growth, like the four seasons. It is because our descendents, mammals had to grow fur and hair during various seasons to create a protective coating, and this cyclicle nature of hair growth persists in humans, even though we don’t have the requirements other mammals have to meet environmental change.
The hairs on our head go through the four cycles of growth: Anagen (growing phase), Catagen (regression phase), Exogen (shedding phase), and Telogen (resting phase). Each follicle cycles on it’s own, there is no seasonal timing for our hair growth, unlike a beaver, for instance, which grows fur thicker in the fall for protections in the winter. The beaver’s hair follicles all cycles together, but humans hair follicles distribute through the four phases and thus on any average day, you will lose 50-100 hair from your head, while replenishing at the same rate to maintain a balanced head of hair. Sometimes, that cycle is off and can coordinate, hair follicles falling into the same phase, thus leading to more shedding at one time. This is the most common cause for women to notice a great amount of shedding that is more than normal. Typically this is a self-limited process that resolves in several months as the hair follicles cycle back into various phases of growth, as opposed to cycling in unison. Sometimes the reasons for hair follicles coordinating their cycles together is apparent, as in the case of stress or health issues, but often there is not definitive connection. Still, the hair follicles tend to redistribute their cycle and get back to a normal pattern, preventing shedding from creating a true ‘bald’ scalp. That the phases of hair growth and the cycling of hair is a mammalian feature, explains the backdrop to our four cycles of growth. That we do not need fur for seasonal protection explains why our hairs cycle at different times and why we do not shed like other mammals do, but there is a great deal more to know about the science of hair growth, and Hair, A Human History, explains this in interesting detail….I recommend it.