Hair on a human scalp grows as a natural bodily process through cell division, growing an average 0.3 millimeters a day, or a total of 1 centimeter per month. Biological timing staggers natural growth and shed rates in a healthy scalp to prevent "molting," or losing it all at once.
The hair shaft is grown from and tethered by a follicle in the skin of the scalp; its hue and shape are dependent on genetic factors that vary from one individual to another. This hair shaft goes through three growth periods before shedding and beginning the cycle again. These phases are initial growth (anagen), slowing down (catagen), and a resting stage (telogen).