The Hair Growth Cycle: Why Hair Sheds and Regrows

Almost everything about hair loss makes more sense once you understand the cycle. Why shedding is normal. Why pattern baldness is gradual rather than sudden. Why a transplanted follicle keeps growing when the ones beside it have stopped. Why results take a year to appear.

It is worth ten minutes.

What hair actually is

The hair you can see is not living tissue. It is a shaft of keratin, a structural protein, pushed upwards by activity in the follicle beneath the skin.

The living part is the follicle itself, and specifically the cells at its base. Everything that determines whether you keep your hair happens there, not in the shaft. This is why products applied to the hair itself can improve how it looks and feels but cannot influence whether it continues to grow.

A healthy adult scalp carries somewhere in the order of 100,000 follicles. Hair grows at roughly a centimetre a month, though this varies with age and between individuals.

The three phases

Each follicle runs through a repeating cycle with three stages.

Anagen, the growth phase. Cells at the base of the follicle divide rapidly and push the shaft upwards. On the scalp this phase lasts several years, commonly quoted as two to seven. At any moment, the large majority of your scalp follicles are in anagen.

The length of the anagen phase is what determines how long your hair can grow. Someone whose anagen phase runs to seven years can grow hair well past their shoulders. Someone with a two-year phase cannot, no matter what they do. This is also why eyelashes and eyebrows stay short: their anagen phase is measured in weeks.

Catagen, the transition phase. A short phase of a few weeks. The follicle detaches from its blood supply and begins to shrink. Growth stops.

Telogen, the resting phase. Lasting a few months. The follicle is dormant and the hair is held in place but no longer growing. At the end of telogen the hair is released and a new anagen phase begins beneath it.

Some classifications add a fourth stage, exogen, describing the actual shedding of the hair as the new one grows in.

Why you shed every day

The important detail is that follicles are not synchronised. Each one runs its own cycle independently.

Because of that, at any given moment a small proportion of your follicles are in telogen and releasing hairs, while the vast majority are in anagen and growing. The result is continuous, gradual shedding rather than periodic mass loss.

Losing somewhere in the range of 50 to 100 hairs a day is unremarkable. Most people never notice it, because 100 hairs out of 100,000 is a rounding error.

Where people do notice is when several days of shedding accumulate visibly at once: after washing, after not washing for a few days, or after brushing following a period of leaving hair alone. That is usually collection, not increase.

See how much hair loss is normal.

What disrupts the cycle

Most hair loss is a disruption to this cycle rather than damage to the follicles themselves. Which part of the cycle is affected determines what the loss looks like.

A large proportion of follicles pushed into telogen at once. This is telogen effluvium. A physiological stressor, significant illness, surgery, high fever, childbirth, rapid weight loss or severe psychological stress, forces many follicles into the resting phase simultaneously. Because telogen lasts a few months, the shed appears two to three months after the trigger, which is why the cause often is not obvious. It is diffuse and it recovers.

Seasonal variation in the telogen proportion. A milder version of the same mechanism, following an annual rhythm rather than a discrete event. See seasonal hair loss.

Progressive shortening of the anagen phase. This is pattern hair loss. Each cycle, susceptible follicles spend less time growing, producing a hair that is finer and shorter than the last. Over many cycles the hair becomes too fine to see. The follicle persists but effectively stops contributing. This is called miniaturisation and it is why pattern loss is gradual and why there is a window during which follicles are still salvageable.

Sustained mechanical tension. Traction alopecia. Constant pull on the follicle causes inflammation and, if prolonged, scarring that destroys it. Reversible early, permanent once scarred. See traction alopecia.

Immune attack on the follicle. Alopecia areata, producing discrete circular patches rather than diffuse or patterned loss.

Interruption of rapid cell division. Chemotherapy targets rapidly dividing cells, which includes anagen follicles. The loss is abrupt and largely reversible once treatment ends, though texture can change.

See when hair loss isn't genetic.

Why a transplanted follicle keeps growing

This is the part that matters most if you are considering surgery.

Pattern hair loss is driven by follicular sensitivity to dihydrotestosterone, or DHT. That sensitivity is not uniform across the scalp. Follicles at the front and crown of a susceptible scalp respond to DHT by shortening their anagen phase. Follicles at the back and sides are largely resistant, which is why men who lose hair on top keep hair around the sides well into later life.

Critically, that resistance travels with the follicle. Move one from the back of the scalp to the front and it continues behaving as it did before, because the property belongs to the follicle rather than to the location.

This is called donor dominance, and it is the entire basis of hair transplant surgery. It is also why the donor area is finite and irreplaceable: only follicles from the resistant zone are worth transplanting.

See male pattern baldness and the Norwood Scale and hair transplant surgery in Sydney: the complete guide.

Why results take so long

The cycle also explains the timeline after surgery, which surprises almost everyone.

A transplanted follicle has been extracted, handled and placed into a new site. That trauma pushes it into telogen. Within two to six weeks it releases the hair it was carrying, which is why most transplanted hairs shed. The follicle stays put.

It then sits in the resting phase for a few months before entering anagen and beginning to produce new hair. That is why there is usually little visible growth for the first four months, why new hairs typically appear between four and eight months, and why the final picture takes twelve to eighteen months.

It is not slow healing. It is a follicle completing a normal cycle.

See hair transplant growth timeline.

The practical takeaways

• Shedding is a normal feature of the cycle, not a sign of loss
• What matters is not how much you shed but whether what regrows is the same calibre
• Diffuse sudden shedding usually reflects a temporary cycle disruption and tends to recover
• Gradual patterned thinning reflects progressive miniaturisation and does not
• The follicle, not the hair, is what treatment and surgery act on
• Surgical results follow the cycle, which is why they take a year or more

Getting it assessed

If you are unsure which of these applies to you, examination under magnification can distinguish diffuse shedding from miniaturisation, which is the distinction that determines what to do next.

A GP referral is required before a cosmetic surgery consultation. Get in touch.

Related reading: how much hair loss is normal, hair loss treatments compared.