Hair Biology

How Your Hair Grows

The hair grows approximately 1 centimeter a month and each person normally loses approximately 70 to 100 hairs a day. Every hair follicle on the scalp goes through three phases of follicular development that repeats over time. These phases are called anagen, which is the growth stage, telogen which is the resting stage, and catagen which is the involutional stage. Most of the hair on the scalp is in the growing phase, which can last between 2 to 6 years. The catagen stage is the shortest and lasts approximately two to three weeks. The telogen phase typically lasts three to four months. This is the phase where new anagen hair emerges replacing the older telogen hairs which can be typically found on the brush after combing.

A variety of factors can influence the hair growth cycles. Restrictive diets, hormonal changes, genetics and excessive emotional or physiologic stress can all cause hair loss. These shorten the anagen phase and prolong the telogen phase. As most individuals grow older, the percentage of hairs in the telogen phase increases which means there is less anagen growing hairs. Hair bulbs from mature patients produce hairs with smaller diameter and less pigmented.