Why Sessions Are Spaced the Way They Are: The Growth Cycle

A client comes back after session one delighted. After session four, quiet and slightly suspicious. Same tech, same settings, same machine — what changed is how many follicles were awake.

Short answer: one treatment can only finish the follicles that happen to be in anagen that day, which is why the interval is set by area rather than by a single house rule. Face 4 weeks. Underarm and bikini 5 to 6. Arms, chest and back 6 to 7. Legs 7 to 8. Nothing past 8 weeks in any area, and when you can't decide, book the short end — then correct by the regrowth you see at the next visit.

That's the grid. The rest of this is why it's shaped that way. Platform-level view sits on our laser hair removal page.

Three Phases, and Only One of Them Is a Target

In anagen the follicle is building. Matrix cells divide, the papilla feeds the bulb, and pigment cells peak — that last detail is the whole business model. Melanin is what your light aims at, and anagen is when there's most of it, sitting right beside the structures you want to damage.

Catagen shuts that down. The StatPearls hair follicle chapter describes matrix division stopping, the lower segment regressing, a club hair forming with a hard white node. Our internal training material adds the operator's bit: capillaries thin, the papilla atrophies, pigment production stops. Weeks, not months.

Then telogen. The follicle has separated from the papilla; what's left is finished tissue awaiting release — StatPearls calls that step exogen. Fire at it and you're heating a shaft no longer plumbed into anything worth killing.

Why a resting follicle shrugs off the pulse

Kolinko, Littler and Cole reported in Lasers in Surgery and Medicine in 2000 that test sites with a low anagen count showed low hair loss compared with high-anagen sites — same device, same operator, different phase mix. Their laser was a Q-switched Nd:YAG, not the diode in your treatment room. That doesn't undo the finding for you: what they were testing was the follicle cycle, not the wavelength. They also described what the laser does to the hairs it does catch — anagen hairs switch rapidly into telogen, then shed.

The number nobody in your room can measure

You cannot eyeball an anagen ratio. Bouabbache and colleagues measured density, growth rate and anagen percentage across axilla, cheek, chin, leg and upper lip in the International Journal of Cosmetic Science in 2019; abundance came out broadly similar across the ethnic groups studied, while the functional numbers tracked the individual and the gender instead. Our own engineering archive keeps a site-by-site table of anagen and telogen percentages, phase durations, density and depth. Good for teaching a class. Useless for predicting the person on your table.

The Shed That Gets Mistaken for a Result

A week or two after session one, treated hairs push out of the skin. Clients see stubble and assume regrowth. It isn't — it's the shedding Kolinko's team described, and it's a good sign. Say it at consultation, not afterwards.

That's also why session one wins the popularity contest: you clear the largest single cohort of anagen hairs the area will ever offer, and for a few weeks the skin looks finished. It isn't. Bouabbache's group put body hair growth rates between roughly 180 and 485 micrometres a day across the sites they measured — slow sites a bit over a millimetre a week, fast ones more than three. Once a follicle restarts, you see it fast.

Between visits: shave. Nothing that pulls the shaft out — that shaft is the target you're waiting on.

Why Session Four Feels Like It Did Nothing

Two things shrink at once

First, arithmetic. Each session clears a share of what's left. That share out of a hundred hairs shows; out of twenty it doesn't. I won't quote you a per-session clearance for this equipment. But note that both published figures in this piece — Bouzari's 78.1% at a 45-day interval, and the 75.07% at six-month follow-up reported by Bacchini and colleagues in Lasers in Medical Science, 2025 — are whole-course numbers, not single-visit ones. Spread either across five visits and session four looks like nothing happened. Draw that arithmetic for the client before session four, not after they ask.

Second, target quality. The hairs that survive a course are finer and lighter. Less pigment, less absorption — the same fluence buys less follicle damage. That 2025 protocol was built around the drift: sixty patients, wavelengths rotated as the surviving hair got finer, a minimum three-session course run until only fine vellus remained. Multi-wavelength, sixty patients — read it as how a course behaves, not a number for your price list.

Don't answer the plateau by cranking energy. Endpoints and cooling logic sit in our walkthrough of how an 808nm session actually runs.

What to say at session four

Photographs, same way every visit, same light, same distance. The client's memory anchors on the shed window, not on where they started. Show them the first frame.

That same paper also reported cycles in the treated area appearing to synchronise after the first three sessions. One study. But if it holds, mid-course is where intervals matter most, because the whole field arrives together.

Setting the Interval by Area

Where the default numbers come from

Our diode treatment protocol for European clinics is blunt: at least four to five sessions, four to seven weeks apart, to maximise the chance of catching hair in anagen. The 2025 protocol ran its first three at four to six weeks, then went by regrowth. Our training notes add the principle — different sites cycle differently, so intervals should differ too. Everything below is clinic convention built on that, not a published schedule.

Stretching an interval costs you, though. Bouzari and colleagues, 2005 compared 45, 60 and 90-day intervals with an 800nm diode on face and neck: mean reductions of 78.1%, 45.8% and 28.7%. Hold onto the site — face and neck, the fastest-cycling area on the list, and nobody has run that comparison on legs. The direction still travels: the longer the gap, the less each visit clears. Booking an area long is a trade, not a free lunch.

A grid for the booking screen

Decision rule before the numbers: when you can't decide, book the short end. Arriving too early costs you one appointment. Arriving too late costs anagen share on every hair in the field.

• Upper lip, chin, cheeks: 4 weeks. Facial hair returns fastest, and clients notice soonest.
• Underarm, bikini: 5 to 6 weeks.
• Arms, chest, back: 6 to 7 weeks.
• Legs: 7 to 8 weeks — and this row is a real choice, not a fact. Leg follicles cycle slowest, so the wait buys a higher anagen share, but on Bouzari's curve the clearance from any one visit at that spacing will sit well below a six-week booking. Seven to eight weeks for the client who'd rather add a session than add trips. Six for the one in a hurry.
• Hard cap: eight weeks, nothing past it, in any area. That's our house rule rather than a published finding — Bouzari tested 45, 60 and 90 days and never eight weeks. We put the cap where their curve turns over, between the 45-day and 60-day marks.

Correct it by looking, not by calendar

Three checks, every visit:

1. Field still bare on the booked date? Too early. No shaft, no pigment, no target — you'd be charging to fire at empty sockets. Push a week and write the new interval on the card.
2. Heavy regrowth well before the date? Tighten by a week next round — floor of four weeks on the face, five on the body, and only if the last session's erythema and swelling settled fully with no pigment change left behind. Darker skin types and recent sun exposure don't get tightened at all; what goes wrong is in our notes on burns, blisters and PIH.
3. Same client, two areas, one appointment? They won't stay in step — a face-and-legs client is two schedules sharing a chair.

Log the interval you actually ran, per client, per area. A few dozen courses in, you'll have your own grid.

What This Does to Your Book

Interval is a commercial number, not just a clinical one. Count it from first session to last, which is one gap fewer than the session count. Four to five facial sessions at four weeks is three to four months on the book, and because hormones keep driving facial growth that client usually rolls into maintenance instead of finishing — the only reason a face fills eight to thirteen slots in a year. Legs, same four to five sessions at seven to eight weeks, run roughly five to seven and a half months for the first course: four or five slots inside the year, then it's over. One is a subscription. The other has an end date. Price and staff them differently.

A few things to build into the process:

• Rebook in the room, before they're dressed. A missed facial appointment costs a whole cycle, not four weeks.
• Sell courses, reassess at three. That's where the plateau conversation belongs, and where the 2025 protocol put its own checkpoint.
• Watch seasonality. Legs sell in spring, and an April start is two or three sessions deep by June. Say so at booking or you'll wear the disappointment.
• On mixed-tone or mixed-coarseness books, a large-spot 808nm diode next to a small-spot long-pulse Nd:YAG gives your tech somewhere to go when the survivors get fine. Configuration questions go to our team.

None of this is medical advice. It's equipment and operations education — your clinical protocol comes from your own licensed supervisor, and undiagnosed lesions get referred, not treated. Compiled by the Pmise applications engineering team from our internal training archive and the papers cited below; it has not been through independent clinical review.

Can we shorten the interval to finish a course faster?

Not usefully. If the follicles haven't re-entered anagen there's no pigmented shaft to absorb the pulse, and you've spent a slot for nothing. Sit inside four to eight weeks depending on the area — short end for face, long end for legs — and adjust by what you see, not by what the client's holiday calendar wants.

Why does the same client respond well on legs and badly on the upper lip?

Different sites cycle differently, and the anagen share at the moment you treat decides the outcome for that visit. The 2000 Lasers in Surgery and Medicine study found low-anagen sites gave low hair loss with the same device (that work used a Q-switched Nd:YAG, so read the finding as a statement about the cycle, not about wavelength). Facial hair has hormonal drivers behind it too, which is why facial courses tend to turn into maintenance rather than finishing.

The client says the hair is growing back a week after treatment. Is the machine underpowered?

Check your endpoint log first, but usually no. Treated hairs get expelled over the following days and weeks, and that looks exactly like stubble. Script it at consultation so the first call your front desk gets isn't a complaint.

How many sessions should we quote per area?

Quote a range and rebook by result. Our diode protocol sets the floor at four to five sessions; the 2025 rotational paper ran a minimum of three, then continued until remaining hair was fine vellus. Areas with a smaller anagen fraction need more visits — stated plainly in our internal training material, and the honest reason a course length can't be promised up front.

1. Martel JL, Miao JH, Badri T, Fakoya AO. Anatomy, Hair Follicle. StatPearls, NCBI Bookshelf - anagen, catagen, telogen and exogen; club hair formation; papilla and bulge
2. Kolinko VG, Littler CM, Cole A. Influence of the anagen:telogen ratio on Q-switched Nd:YAG laser hair removal efficacy. Lasers in Surgery and Medicine, 2000;26(1):33-40
3. Bouabbache S, Pouradier F, Panhard S, Chaffiotte C, Loussouarn G. Exploring some characteristics (density, anagen ratio, growth rate) of human body hairs. Variations with skin sites, gender and ethnics. International Journal of Cosmetic Science, 2019
4. Bacchini G et al. Medical laser hair removal: a new rotational approach. Lasers in Medical Science, 2025 - three initial sessions at 4-6 week intervals, cycle synchronisation, progressive miniaturisation of remaining hair
5. Bouzari N, Tabatabai H, Abbasi Z, Firooz A, Dowlati Y. Hair removal using an 800-nm diode laser: comparison at different treatment intervals of 45, 60, and 90 days. International Journal of Dermatology, 2005 - face and neck