In simplified terms, human hair has a two-part structure. Anchored deep in the scalp is the living part of the hair: the follicle. At the bottom end of the follicle new hair cells are created. From a biological point of view, the visible part of the hair, the shaft, consists of dead cells. Hairs do not grow continuously but in cycles. Phases of hair formation, rest and regression follow each other in a rhythmic progression. As each follicle follows its own rhythm, which does not match the cycles of the other follicles, hair production is said to be asynchronous. This is an important aspect of human hair, as it ensures that the head always has a covering of hair.

The growth cycle of a hair consists of three phases:
1) The anagen (growth) phase, which lasts two to six years. 2–3) The two- to four-week catagen (transitional) phase: During this time, hair growth ceases and the hair follicle becomes shorter. 4) The three- to four-month telogen (rest) phase: No hair growth occurs during this time. 5) Start of a new anagen phase: A new hair bulb forms and a new hair begins to grow. The old hair is gradually pushed up and out. Over the course of a person’s life, a single hair root can thus produce ten to twelve hairs in succession.