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The Hair Growth Cycle: Why Laser Takes Multiple Sessions
Short answer: Your hairs do not all grow at the same time. Each one passes through three phases: growth (anagen), transition (catagen) and rest (telogen). Laser is most effective on hair in the anagen phase, when the hair is attached to the root and pigment levels are high. Because only part of your hair is in that phase at any moment, each session affects that part. You then have to wait for the rest to take its turn, which is exactly why session intervals and a series of sessions exist.
The Three Phases of the Hair Cycle
Every hair follicle is like a tiny factory running on its own clock. Two neighbouring hairs can be in completely different phases. The cycle has three steps:
| Phase | What happens | Laser effect |
|---|---|---|
| Anagen (growth) | The hair is actively growing, firmly attached to the root, with high pigment. | Most effective: light reaches the root via the pigment. |
| Catagen (transition) | Growth stops and the hair begins to detach from the root. A short phase lasting a few weeks. | Limited effect. |
| Telogen (rest) | The hair is no longer attached and waits to shed; the follicle rests before making a new hair. | Ineffective: heat cannot travel to the root. |
When telogen ends, the follicle returns to anagen and starts producing a new hair. Those are the hairs the laser will catch at your next session.
Why Is One Session Never Enough?
During any session only a share of the hair in an area is in anagen; that share varies by area and by person. The laser affects that share. Resting hairs simply do not “see” the light and start growing again weeks later. That is why laser hair removal is planned as a series: each session catches a new group of hairs that has become active.
For most areas, 6–8 sessions is a common range. The face, where hormones play a bigger role, may need more.
Why Do Intervals Differ by Area?
The hair cycle does not run at the same speed everywhere on the body. Facial hair cycles faster; on the legs and back the resting phase lasts longer. Session intervals follow that rhythm:
- Face (upper lip, chin, beard line): usually 4–6 weeks
- Underarms and bikini: usually 5–8 weeks
- Arms, legs, back: usually 6–10 weeks
Booking sessions closer together does not speed things up, because hair that has not yet entered anagen cannot be caught. Leaving too long a gap, on the other hand, misses the cycle and stretches out the whole course.
What You Will Notice After a Session
Treated hairs do not vanish straight away. Usually within 1–3 weeks you will notice some of them being pushed towards the surface and shedding on their own. Many people's first worry is: “The hair is still coming out, did the laser not work?” We cover this shedding phase in a separate article: why is hair still coming out after laser?
What Affects the Hair Cycle
- Hormones: puberty, pregnancy, menopause and some hormonal conditions (such as polycystic ovary syndrome) can trigger new hair growth. If hormonal hair growth is suspected, a medical assessment is recommended before laser.
- Hair colour and thickness: dark, coarse hair carries more pigment and responds better. Very light, white or red hair may respond only to a limited extent.
- Waxing and tweezing: pulling hair out by the root disrupts the cycle and removes the laser's target. Only shaving should be used during a laser course. See our switching from waxing to laser guide.
Why the Device Matters
The hair cycle is biology, and no device can skip it. But how much of the anagen hair is effectively targeted at each session depends on wavelength, settings and skin protection. The Megatron 4D Diode we use combines 755, 808, 940 and 1064 nm wavelengths in one handpiece, so settings can be adjusted from fine, shallow hair to coarse, deep-rooted hair. We compare technologies in our Alexandrite vs diode article.
Summary: Making the Hair Cycle Work for You
- Stick to your session interval; do not bring sessions forward or let them slip without reason.
- Only shave between sessions.
- Do not pull out shedding hairs; leave them alone.
- Protect your skin from strong sun; a tan limits the settings.
- If you notice a hormonal change, tell us and your doctor.
To see your personal session interval and an estimated number of sessions, start with a free hair analysis. For all areas and how treatment works, see our laser hair removal page.
This content is for general information and does not replace medical advice. If you suspect hormonal hair growth, consult your doctor. Results may vary from person to person.
Frequently Asked Questions
Answers to the most frequently asked questions about this content
What is the hair growth cycle?
It is the sequence of growth (anagen), transition (catagen) and rest (telogen) that every hair goes through. Neighbouring hairs can be in different phases.
Which phase does laser work on?
It is most effective in anagen. In this phase the hair is attached to the root and pigment is high, so heat can travel to the root.
Why can't laser be done in one session?
At each session only part of the hair is in anagen. Resting hairs can only be caught at later sessions.
Why do intervals differ between areas?
The hair cycle runs at different speeds in different areas. Intervals are usually 4–6 weeks on the face and 6–10 weeks on the legs and back.
Will closer sessions shorten the course?
No. Hair that has not yet entered anagen cannot be caught at an early session, so the course does not get shorter.
Do hormones affect the hair cycle?
Yes. Pregnancy, menopause and some hormonal conditions can trigger new hair growth. If you suspect hormonal hair growth, a medical assessment is recommended.
