The hair on your head and body undergoes a dynamic, repeating process. Each hair follicle, the small pocket in the skin from which a hair grows, operates on its own timeline. This cycle ensures hair is continuously renewed and involves distinct periods of growth, transition, and rest. This sequence dictates the presence, length, and thickness of hair across the body.
The Four Phases of Hair Growth
The hair growth cycle has four distinct phases, with the first being the anagen or growth phase. This is the active period where cells in the root of the hair are rapidly dividing, forming a new hair shaft. For scalp hair, this phase is long, lasting from two to seven years. The duration of the anagen phase determines the maximum length a hair can achieve. About 85-90% of your scalp hairs are in this state.
Following the growth phase is the catagen or transition phase, which is significantly shorter, lasting for approximately ten to fourteen days. During this time, the hair follicle shrinks and detaches from the dermal papilla, which supplies it with nutrients. This process cuts the hair off from its nourishment, halting any further growth.
Once the transition is complete, the hair follicle enters the telogen or resting phase. The hair strand is no longer growing but remains anchored in the follicle for about three months. While the old hair rests, a new hair often begins to form beneath it, preparing to start its own anagen phase. An estimated 10-15% of all hairs on your head are in the telogen phase at any one time.
The final stage is the exogen or shedding phase, an extension of the resting phase where the hair is released from the follicle and falls out. This shedding is a natural process, and it is normal to lose between 50 and 100 hairs each day. As the old hair is shed, the follicle re-enters the anagen phase to grow a new hair.
Factors That Influence the Cycle
The precise timing and health of the hair growth cycle are not uniform and can be influenced by a range of internal and external factors.
- Genetics play a foundational role, largely determining the length of the anagen phase and influencing hair characteristics like thickness and texture. These inherited traits are a primary reason why some individuals can grow their hair much longer than others.
- Hormonal fluctuations from events such as pregnancy, menopause, or thyroid disorders can disrupt the normal rhythm, often pushing a larger number of hairs into the telogen phase. This can lead to increased shedding, a condition known as telogen effluvium.
- Nutrition is another important factor, as the rapid cell division during the anagen phase requires adequate energy and nutrients. Deficiencies in vitamins and minerals can impair the follicle’s ability to produce strong, healthy hair.
- Significant physical or emotional stress can interrupt the cycle. Severe illness or major stress can trigger telogen effluvium, where many hairs are shifted into the resting phase, leading to diffuse shedding a few months later.
- The aging process also naturally alters the hair growth cycle. As people get older, the anagen phase shortens, which means hair may not grow as long as it once did and can appear thinner over time.
Cycle Variation Across the Body
A key reason for the difference in hair length across the body is the variation in the duration of the anagen phase. While scalp hair has an anagen phase that can last for several years, the hair on other parts of the body has a much shorter growth period. This difference explains why scalp hair can grow long, while other body hairs remain short.
For example, the hair that forms your eyebrows and eyelashes has a brief anagen phase, lasting only about 30 to 45 days. This short window for active growth is why these hairs do not grow long like scalp hair. This rapid cycle ensures they remain at a consistent, functional length.
Similarly, the hair on your arms and legs also has a significantly shorter anagen phase compared to scalp hair. The growth phase for these hairs might last for a few months before the follicles enter the resting and shedding stages. This genetically programmed duration dictates the maximum length of hair in any specific area.