Hair, a prominent feature of the human body, serves various functions beyond its aesthetic role. It provides protection against the sun’s ultraviolet rays and offers insulation from cold temperatures. Hair also contributes to sensory perception, with nerve fibers at the base of each follicle detecting movement on the skin’s surface. While hair appears to grow continuously, it undergoes a natural cycle of growth, cessation, and shedding.
The Hair Growth Cycle
Hair growth is a cyclical process, with each individual hair follicle operating independently through distinct phases. This cycle ensures that not all hairs fall out simultaneously, maintaining a relatively stable hair density. The complete cycle involves four main stages: anagen, catagen, telogen, and exogen.
The anagen phase is the active growth period, where cells in the hair follicle rapidly divide to produce new hair. This is the longest phase, lasting approximately two to eight years for scalp hair. During this time, hair can grow around 1 to 2 centimeters per month. Approximately 85% to 90% of scalp hairs are typically in this growth phase.
Following the anagen phase, hair enters the catagen phase, a brief transitional period. This stage lasts about two to three weeks, during which hair growth slows significantly. The hair follicle shrinks and detaches from the dermal papilla, which supplies blood and nutrients. This detachment prepares the hair for its resting stage.
The telogen phase is a resting period where the hair remains in the follicle but no active growth occurs. This phase typically lasts for two to three months. Around 9% to 15% of scalp hairs are in the telogen phase. During this stage, a new hair often begins to form beneath the resting hair.
The final stage is the exogen phase, characterized by the shedding of the old hair. This phase is distinct from the telogen phase. During exogen, the old hair detaches and falls out. Losing 50 to 100 hairs daily during this phase is considered a normal part of this natural renewal process.
Factors Influencing Hair Growth and Loss
Beyond the natural hair growth cycle, various internal and external factors can disrupt this process, leading to altered growth patterns or increased hair loss. Understanding these factors helps in recognizing deviations from typical hair behavior.
Genetics
Genetics play a significant role in determining hair characteristics and susceptibility to certain types of hair loss. Genetic factors influence the length of the anagen phase and the likelihood of experiencing pattern baldness.
Hormonal changes
Hormonal changes can notably impact hair follicles. Fluctuations in hormones, such as androgens, thyroid hormones, or those occurring during pregnancy or menopause, can alter hair growth. An imbalance can shorten the anagen phase, leading to thinner hair or increased shedding.
Nutritional deficiencies
Nutritional deficiencies can impair hair growth by depriving follicles of necessary building blocks. A lack of essential vitamins and minerals, including iron, zinc, or biotin, can disrupt the hair cycle.
Stress
Periods of significant physical or emotional stress can also affect hair growth. Severe stress can prematurely shift a large number of hairs from the growing phase into the resting phase. This can result in a temporary but noticeable increase in hair shedding, often occurring a few months after the stressful event.
Medical conditions and medications
Certain medical conditions and medications can interfere with the hair growth cycle. Autoimmune diseases, infections, or conditions like thyroid disorders can impact hair follicles. Some medications, such as chemotherapy drugs or blood thinners, are also known to cause hair loss as a side effect.
Aging
As individuals age, natural changes occur in the hair growth cycle. The anagen phase may become shorter, and hair follicles can produce finer, less robust hair.
Common Hair Loss Patterns
When hair stops growing or sheds excessively due to various underlying factors, it often manifests in distinct patterns. These observable manifestations differ from the normal, cyclical shedding.
Androgenetic alopecia
Androgenetic alopecia, commonly known as male or female pattern baldness, is a widespread form of hair loss. This condition is primarily influenced by genetics and hormones. In men, it typically presents as a receding hairline and thinning at the crown, while in women, it often appears as diffuse thinning over the scalp.
Telogen effluvium
Telogen effluvium is characterized by widespread, temporary hair shedding across the scalp. This pattern usually occurs after a significant physical or emotional stressor, illness, or hormonal shift. A large number of hairs prematurely enter the telogen (resting) phase, leading to increased shedding several months later. This condition is often temporary, with hair regrowth expected once the underlying cause is addressed.
Alopecia areata
Alopecia areata is an autoimmune condition where the body’s immune system mistakenly attacks healthy hair follicles. This leads to hair loss that typically appears as smooth, round patches on the scalp or other parts of the body. The extent of hair loss can vary significantly among individuals.
Traction alopecia
Traction alopecia is a type of hair loss caused by consistent pulling or tension on the hair follicles. Hairstyles that tightly pull the hair, such as braids, ponytails, or weaves, can lead to this condition.