As you grow, your hair goes through a cycle of growth and loss and regrowth. Human hair is random, not seasonal or cyclical like other mammals. At any given time, hair will be at one of three growth stages, with every hair at a different stage.
To first understand the growth stages of hair, we should look at what makes up our hair. There are two distinct structures to hair: the follicle and the shaft. The hair follicle is a tunnel-like segment of the outer layer of skin that extends down into the second layer of skin called the dermis. At the base of the hair follicle is the dermal papilla which contains tiny blood vessels that provide nourishment to the cells. Surrounding the papilla is the bulb, the living part of the hair. The cells of the bulb divide faster than any other cell in the body, dividing every 24 to 72 hours. Two sheaths surround the hair follicle, an inner and outer sheath, which protect and form the growing hair. The hair shaft is made of keratin, a hard protein, and is composed of three layers. The inner layer is medulla, the middle layer is the cortex and the outer layer is the cuticle. The cortex makes up the majority of the hair shaft and holds the hair’s pigment with the medulla.
The first stage of hair growth is referred to as the anagen phase. The anagen stage is the growth phase, usually lasting between two to eight years. During this stage, the growth cells in the papilla rapidly divide, producing the hair shaft which becomes keratinized as it pushes up and out of the follicle. During the same time, the follicle grows down deeper into the dermis to get nourishment. People with long anagen growth rates are able to grow longer hair than those with shorter anagen phases. Approximately 85% of all hairs are in the growth phase at one time.
Following the anagen phase, the hair goes through a transitional stage, referred to as the catagen stage. This stage usually occurs over a brief two to four weeks. During the catagen phase, the hair follicle shrinks and breaks away from the dermal papilla. As the hair follicle separates from the papilla, the bulb detaches from the blood supply and the hair shaft is pushed up as the follicle disintegrates. About three percent of all hairs are in this stage at any given time.
The follicle then goes into the resting phase known as the telogen stage. The telogen phase lasts from two to four months, during which the hair does not grow, but remains attached to the follicle. In this phase of hair growth, the follicle and the dermal papilla below are completely at rest. Approximately 10-15 percent of all hairs are in the telogen phase at one time. As the resting phase comes to an end and new hair is formed, old telogen hairs are pushed out and lost. About 50-100 telogen hairs are lost at this time due to the growth process, which is considered normal hair shedding.
Once the telogen phase ends, the hair growth cycle is complete. Hair will go back into the anagen stage and the process begins again.