Hair is made up of proteins called keratins, and it grows from follicles underneath the skin. Although every strand of hair on the human body goes through the hair growth cycle, each hair grows independently of the other.
The hair growth cycle occurs in three main phases: the Anagen (growing) phase, which lasts 2 to 6 years, is when the hair is firmly attached to the hair root; the Catagen (transitional) phase lasts a few days and is when the hair stops growing and forms the basis for the next hair; and the Telogen (resting) phase which lasts 3 to 4 months and is when the hair sheds and causes the stem cells to move deeper in the dermis to start a new anagen phase.
People are born with approximately 5 million follicles on their body, and about 100,000 follicles on just the scalp. Regardless of where the hair is located on the body, all hairs undergo a similar growth cycle.