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Contrary to popular belief, human hair does not grow all at once. Rather, each individual strand is constantly undergoing one of three phases to determine whether or not it should grow and shed. The anagen phase is the part of the hair growth cycle when the hair enters a state of active growth. In other words, it is the only time new hair is growing. Most dermatologists agree that this time period lasts anywhere from three to five years. When using medications such as Rogaine the anagen phase is lengthened which helps hair to grow longer and fuller.
During the anagen phase, the following transformations occur under the scalp:
It is important to note that if the dermal papilla fails to make connection with the hair bulb, then it cannot provide nourishment, and the hair will not be able to grow.
As far as their respective mechanisms of growth are concerned, body hair grows in exactly the same fashion as scalp hair – that is to say, each strand of hair on the body is also going through one of three phases of development, and not at the same time. However, body hair remains in the growing phase for a much shorter duration than that of scalp hair. This is why hair on top of the head grows remarkably longer than hair on the chest, arms, legs, etc.
The duration of the anagen phase is largely determined by genetics. For example, those of Asian descent typically have hair that stays in this growth phase for a longer duration than other ethnicities. In extremely rare cases, body hair remains in an active state of growth for as long as hair on the scalp due to genetic disorder. In other rare cases, scalp hair remains in the phase for as long as body hair, resulting in hair that does not grow more than a few inches long. External factors, such as environmental toxins and extreme stress, can also affect phase length.
Wearing hats and hair ties will not affect the hair growth cycle whatsoever. Pulling your hair too tight, however, can result in hair loss by way of traction. Still, this will not prevent your hair from going through the three growth cycles as normal unless the traction is consistent over a number of years. Likewise, shaving your hair will have no impact on the actual rate of growth. In other words, hair will continue to cycle through the three stages of growth regardless of shaving, and will not grow in longer or thicker as a consequence. Shaving does not affect any of the growth mechanisms that occur under the scalp, which are largely controlled by hormones.