How Hair Cycles; Why Human Hair Is Unique

Most hair-bearing mammals shed in waves which cycles are often seasonally driven. In rats and mice for example, hair is shed in a cephalocaudal pattern beginning at the tip of the nose and progressing rearward toward the tail.  In humans, each individual hair cycles individually -- this distinction is known as a mosaic pattern.  Scalp hairs typically grow in the anagen portion of the cycle for two to five years then transition to a telogen resting phase where the follicle sheds its hair and does not produce a new hair for 90 days.