The hair cycle, explained

What a follicle is

Each hair grows from a follicle: a small pocket in the skin of your scalp. At the base of the follicle, cells divide and build the hair, which is pushed up and out through the skin. The follicle decides how thick the hair is and how long it keeps growing.

A follicle does not grow one hair forever. It works in cycles. It grows a hair for a long time, stops, rests, lets the hair go and then starts a new one.

Your follicles do not keep the same timetable. At any moment some are growing, some are changing and some are resting. That is why you never lose all your hair at once, the way some animals shed their coat in spring.

The three phases

Doctors use Greek names for the phases: anagen, catagen and telogen. We use plain ones: growing, changing and resting.

Growing (anagen)

This is the long, active phase. On the scalp it usually lasts 2 to 7 years. Its length sets the longest your hair can grow. That is why some people can grow hair to the waist, while others find it never gets far past the shoulders.

At any moment, about 85–90% of the hairs on your scalp are in this phase.

Changing (catagen)

At the end of the growing phase, the follicle stops making hair and shrinks back. This short change lasts about 2 to 3 weeks. Only about 1–2% of your hairs are in it at any time.

Resting (telogen)

The hair now sits in the follicle without growing, for about 3 months. About 10–15% of your hairs are resting at any moment.

Near the end of the rest, a new hair starts to grow below the old one. The old hair comes loose and falls out, often when you wash or brush. Then the cycle starts again in the same follicle.

Fig. 1 At any moment, most hairs on a healthy scalp are growing.1 figure = 1 hair

These proportions are averages. They vary between people, with age and across different parts of the scalp. They still give a good picture of what normal looks like.

Why 50 to 100 hairs a day is normal

Every hair you lose has reached the end of its resting phase. About one hair in ten is resting at any time, and each rest lasts about 3 months. So a steady trickle of hairs finishes resting every single day.

Losing about 50 to 100 hairs a day is that trickle. Your scalp holds a very large number of hairs, so this is a tiny share of them. Most of the hairs you lose are already being replaced by new ones growing underneath.

Some everyday things make normal loss look bigger than it is:

Wash days. Loose hairs collect between washes and come out together. If you wash every few days, you see a few days' worth at once.
Long hair. The same number of hairs makes a bigger pile when each one is long.
Brushing and tying your hair. These catch hairs that were already loose. They do not cause the loss.

What matters is a change from your own normal. If you find clearly more hair than usual for weeks, or your hair looks thinner, the cycle may have shifted. The two most common shifts work in very different ways.

When too many hairs rest at once

Sometimes a shock to the body pushes a large group of growing hairs into the resting phase at the same time. Common triggers include an illness with high fever, giving birth, surgery, a crash diet and hard emotional stress.^1,2 Doctors call the result telogen effluvium: a heavy shedding of resting hairs.

At first nothing seems to happen, because the hairs rest quietly in their follicles. Then their rest ends at about the same time, and they fall together. That is why the shedding usually starts about 2 to 4 months after the trigger.^1,2 By then, many people have forgotten the event that caused it.

Doctors have described several ways this can happen. Growing hairs can be released into rest early, or the growing phase itself can be cut short.³ In both cases the follicles are still there; only their timing has changed. When the trigger is found and removed, this kind of shedding settles by itself.²

You can read more in Shedding after stress, illness or birth.

When the growing phase gets shorter

Pattern hair loss works differently. Doctors call it androgenetic alopecia, and it is very common in both men and women.⁴

In people who are genetically sensitive, the hormone DHT acts on follicles on the top and front of the scalp. With each new cycle, the growing phase gets shorter and the follicle gets a little smaller. Doctors call this shrinking miniaturisation.

A smaller follicle makes a thinner, shorter and paler hair. Because each hair grows for less time, a larger share of hairs is resting at any moment. The pause before a new hair appears can also get longer.

So hair is not suddenly lost. It is slowly replaced by weaker hair, and over years the scalp shows through more. That is why pattern hair loss looks like thinning rather than shedding: you may not see more hair in the drain, but the hair you have covers less.

You can read more in Pattern hair loss, explained.

Why any treatment takes months

The cycle sets the pace for every hair treatment. A treatment can only act through the cycle. It may help a follicle stay in the growing phase longer, or start a new hair sooner. Then that new hair has to grow long enough for you to see it. That takes months, not weeks.

This is why the 2017 European guideline on pattern hair loss advises judging treatments such as minoxidil and finasteride after about 6 months.⁴ The same guideline says to warn people that minoxidil can cause shedding in the first 8 weeks.⁴ This is resting hairs being pushed out as the cycle restarts, and it is expected.

Fig. 2 Six months is the earliest point to judge a treatment such as minoxidil.1 square = 1 month

The cycle also explains what happens when a treatment stops. After stopping minoxidil, extra hair loss usually starts about 3 months later,⁴ which is about the length of one resting phase. For finasteride, the guideline says you need to keep taking it to keep the effect.⁴ In short, the gains last only as long as the treatment does.

The same patience applies after a bout of shedding. Once the cause is gone, new hairs grow back at the normal pace, and it takes months before they add visible fullness.

A practical tip: take a photo of your hair every month or two, in the same place and the same light. Day to day, the mirror is a poor judge of slow change. Photos taken months apart are a much fairer one.