Understanding the life cycle of a hair helps to explain why shed hair ends up in your hairbrush, and why not all the hairs on your head are the same length.
The Anagen or Growing Phase of a Hair
The anagen phase of hair growth is the longest of the three phases, usually lasting between two and six years. All other factors being equal, a longer anagen phase allows a person to reach a longer terminal length – the maximum length hair can grow before it naturally sheds out. In reality, diet, hormones, and mechanical and chemical damage to hair can affect how long a person can grow her hair.
During the anagen phase, a hair grows as dead cells of keratin are pushed out through the scalp. The sebaceous glands produce an oily substance called sebum, which helps to condition and protect the hair. At any one time, approximately 85% of the hairs on a human head are in the anagen phase.
The Catagen or Transitional Phase of Hair Growth
Once the anagen phase is complete, the hair enters the catagen or transitional phase. The sebaceous glands produce less sebum and the hair stops growing. During this time a club hair is formed as part of the hair follicle attaches to the hair shaft. This prevents the hair from receiving any more nourishment via the blood supply. After two weeks, the hair begins the telogen phase.
The Telogen or Resting Phase of Hair Growth
The telogen phase last for approximately three months. During that time the hair “just sits there”, still attached to the head but not growing. The hair follicle rests from producing keratin. At this point the club hair is fully formed – you can tell the difference between a shed and a broken hair by looking for a white, hard lump on the root end of the hair. Occasionally these club hairs are mistaken for nits.
After the resting phase is complete the hair sheds out and the cycle begins again. Humans lose approximately 100 hairs a day as some follicles complete the telogen phase. Fortunately, unlike moulting animals, our hairs enter different phases at different times so the shed is unnoticeable! People with thick, dark or long hair often seem to shed more hair, but this is simply an optical illusion – three waist-length hairs look like a lot more hair than three inch-long hairs.
During pregnancy, hormones lock a woman’s hair into the anagen stage or growth. As a result, several month post-partum a higher percentage of hairs than normal may suddenly enter the catagen, followed by the telogen phases. The resulting noticeable hair loss is known as telogen effluvium.
