how hair grows - 'the hair cycle'

To understand how changes can occur to your hair volume it is necessary to first understand the way hair grows.

Each scalp hair is attached to the scalp via a 'follicle' and there are between 100,000 and 350,000 hair follicles on the human scalp. Each follicle grows its hair for an average of 1000 days (3 years) and then rests for a period of around 100 days (3 months). This pattern of active growth followed by the 'resting' period varies significantly from person to person and is influenced by age, diet and our state of health.

The length of hair that you are able to grow is controlled by the duration of the growing phase. If, for example, you have a short growing phase of 600 days, then the hair will grow to approximately 198mm: – that is 600 days at 0.33mm per day of growth. With very long growth phases the hair can grow down to your feet!

Each hair follicle acts independently so while one hair may be growing, the adjacent follicles may be in the resting phase. As a result, humans do not actually have recognised moulting periods unlike birds and some animals.

We all lose some hair naturally each day when we brush, comb or shampoo and as long as new hairs are being produced at the same rate as those falling out, there will be no difference in hair volume. However if the rate of shedding exceeds production the net result is hair loss. For example: if an individual has been losing 50 hairs per day and this increases to 100, twice as many hairs would be observed when combing or shampooing. Whilst 100 hairs are still within the normal range, for this individual it represents a worrying 100% increase in lost hair. Consequently the hair on their head may begin to feel thinner to them although this may not be obvious to anyone else.