1. Anagen (Growth Phase)
The active phase of hair production. Matrix cells in the hair bulb divide rapidly, pushing the hair shaft up and out. In the scalp, this phase lasts 2 to 7 years and determines the maximum length of hair. Approximately 85-90% of scalp hair is in anagen at any given time [1].
2. Catagen (Transition Phase)
A short regression phase lasting 2-3 weeks. Cell division stops, and the follicle shrinks. The lower part of the follicle is destroyed, and the dermal papilla breaks away to rest below.
3. Telogen (Resting Phase)
A dormant period lasting 3 to 4 months. The hair follicle is completely at rest, and the club hair is fully formed. About 10-15% of hairs are in this phase.
4. Exogen (Shedding Phase)
Sometimes classified as part of telogen, this is when the old hair detach and falls out, typically aided by the new hair growing beneath it. Losing 50-100 hairs per day is considered normal physiological shedding.
Clinical Relevance
Disruptions to this cycle lead to hair loss. For example, Telogen Effluvium occurs when a stressor shifts a disproportionate number of follicles into telogen simultaneously. Androgenetic Alopecia involves a progressive shortening of the anagen phase.