If you’re experiencing hair loss problems, the first thing you need to understand is how the hair cycle works.

Hair growth occurs in phases and is dictated by genetics. That is why scalp hair grows long but body hair doesn’t; genetics dictate that the growth phase for scalp hair is much longer than for body hair. So how long is a normal growth phase for the hair on your head?

All hairs (and hair follicles) go through a cycle of growth (anagen), transition (catagen) and rest (telogen) before the hair is shed from the hair follicle, to be replaced (hopefully) by a new hair. For scalp hair, the growth phase might last four years or longer, whereas for body hair, the growth phase is two to three months. For all hair, catagen lasts for about a week and the resting phase is two to three months, after which the hair falls out.

There are many factors that can affect the hair cycle, and abnormal hair loss usually occurs when the growth phase of hair is interrupted. So for example, a high fever can cause growing hairs to pass prematurely into the resting phase. The hairs then stay in the resting phase for two or three months before they fall out and are replaced by new hairs. This type of hair loss is known as diffuse hair loss (or telogen effluvium) and is caused by a change or imbalance circulating through the blood and affecting the hair.

Sympathetic nerve activation (involved in the stress response) can also disrupt the growth cycle and influence the hair follicle. These nerves can trigger hair loss and play a part in hair turning gray as a result of stress.

A viral infection or extreme stress can also set off an autoimmune reaction that causes alopecia areata, where white blood cells attack the hair bulb and cause the hair and hair follicle to pass prematurely into the resting phase. In these cases, the resting phase may only last a few days before the hair falls out, often (but not always) in circular patches. With alopecia areata, the hair is not quickly replaced by new hairs so empty hair follicles can be observed. A general rule of thumb is that if hair loss is patchy, the cause involves the immune system or the sympathetic nervous system rather than the circulatory system.

With genetic hair loss in men and women, the growth phase of some or all the hairs in the affected area becomes progressively shorter so that the hairs themselves gradually become finer and shorter and, especially with men, may eventually disappear. This process can be influenced by male sex hormones circulating in the blood, white blood cells attacking the ‘bulge’ region of the hair follicle where the stem cells producing hair are located, and free radicals that can damage cells.

Hair is normally held firmly in the hair follicle by the interlinking of the hair’s cuticle (composed of overlapping cells, like shingles on a roof) with the cuticle of the inner root sheath, which lies in the opposite direction to the hair’s cuticle. With alopecia areata, this interlinking of the two cuticles becomes disturbed and causes the hair to easily loosen from the follicle within a few days of the hair going into the resting phase. There is another hair loss problem called loose anagen syndrome, where the interlinking of the cuticles is permanently weakened so that hairs easily fall out when shampooing or brushing. If a young blonde girl comes into the clinic and her hair is falling out easily, loose anagen syndrome is something I immediately suspect as it is more common in younger women with blonde hair.

Sometimes people pull out their own hair, a problem referred to as trichotillomania. When the hair is pulled out, the follicle passes into the resting phase and remains there for the normal two or three months before entering a new growth phase – so new hair appears about two or three months afterwards. I have spoken with parents who worry that their children’s hair, pulled out at school, is not growing back; I explain that it will grow back but that it will take at least three months. 

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