Hair Growth Cycle Phases and Laser Hair Removal Timing
The hair growth cycle consists of four distinct phases—anagen (growth), catagen (regression), telogen (rest), and exogen (shedding)—with laser hair removal being most effective during the anagen phase when melanin is concentrated in the actively growing hair shaft.
The Four Phases of Hair Growth
Anagen Phase (Growth Phase)
- Highly mitotic phase characterized by active production of the hair shaft from the hair follicle 1
- This is the longest phase where hair follicles are actively dividing and producing pigmented hair shafts 1
- Blood flow and growth factors promote the telogen-to-anagen transition, initiating new hair growth 2
- Epidermal growth factor (EGF) functions as a biologic switch that must be "turned on" for hair follicles to enter anagen phase 3
Catagen Phase (Regression Phase)
- Apoptosis-driven regression phase where the hair follicle undergoes involution 4
- The follicle begins to detach from its blood supply and regresses 1
- This is a transitional phase between active growth and rest 4
Telogen Phase (Resting Phase)
- Quiescent phase where approximately 9% of hair follicles reside at any given time 1
- The hair follicle remains dormant but the hair shaft is still firmly anchored 5
- Telogen clubs retain cellular elements of the outer root sheath that distinguish them from exogen hairs 5
Exogen Phase (Shedding Phase)
- Distinct shedding phase that was previously assumed to be part of telogen but is now recognized as separate 6
- The largest peak of shedding is coupled to anagen, not telogen 6
- Exogen hairs are passively retained within the follicle with minimal cohesion forces, unlike anagen and telogen hairs which are firmly anchored 5
- The final shedding step involves a specific proteolytic process with intercellular separation of mooring cells 6
Optimal Phase for Laser Hair Removal
Laser hair removal targets melanin in the hair shaft and follicle, making anagen phase the optimal treatment window because:
- During anagen, the hair shaft is actively connected to the follicle with maximum melanin concentration
- The hair is firmly anchored to the follicle, allowing effective energy transfer to destroy the follicular unit 5
- In telogen and exogen phases, the hair-follicle connection is weakened or absent, reducing treatment efficacy
Clinical Implications
Factors Promoting Anagen-to-Telogen Transition (Hair Loss)
- Inflammation, hormonal changes, stress, nutritional deficiency, poor sleep quality, and cellular division-inhibiting medications all accelerate transition to telogen 1
Factors Promoting Telogen-to-Anagen Transition (Hair Growth)
- Increased blood flow, direct follicular stimulation, and growth factors promote return to active growth 2
- Platelet-rich plasma (PRP) treatments accelerate telogen-to-anagen transition by increasing proliferative activity of hair follicle cells 2
Common Pitfall
The asynchronous nature of human hair growth means only a portion of follicles are in anagen at any time, which is why multiple laser treatment sessions spaced 4-8 weeks apart are necessary to capture different follicles as they enter anagen phase.