You may spend plenty of time admiring your gorgeous curls or fresh blowout, but have you ever stopped to wonder how all those hairs on your head grow? Like, what is actually happening underneath the surface to make your strands longer and longer? 

As it turns out, hair growth is cyclical–meaning the pores on your scalp are constantly growing hair, shedding, then growing more hair. 

The anatomy of a hair

In order to understand the cycle of growth, let’s first look at the basic anatomy of a strand of hair.

 The part of your hair that you see growing out of your head is called the hair shaft, aka, the dead part of the hair. Underneath the surface, past the sebaceous gland (where oil is produced) is the living part of your hair that cycles through growth, and this is called the hair follicle. At any given time, we have somewhere between 110,000 and 150,000 hairs on our heads!

The phases of your hair growth cycle

The hair growth cycle is broken down into four phases—two active and two inactive:

Anagen: This is your active growth phase, where the hair follicles are actively growing and getting longer. On a typical scalp, you can have anywhere from 70-90% of your hair follicles in the anagen phase at a given time, which means most of the hair is actively growing! This phase can last anywhere from two to six years. During that time, hair grows an average of one centimeter a month, or four to six inches a year.

Catagen: This is the inactive phase that comes after a hair follicle has stopped growing, and is also called a regression phase. The root of the follicle, though it’s still anchored to the scalp, starts to detach. This process usually lasts around two weeks; typically, less than 1% of hair follicles are in this phase at a given time.

Telogen: This is your cycle's other inactive phase, often referred to as the resting phase. Although the hair stays attached to the scalp, the follicle cells begin signaling to initiate the next anagen phase. This means that even when your hair follicles are preparing to fall, on a typical healthy scalp, they are already signaling for more hair to start growing in their place. This resting phase typically lasts about three months; about 10-30% of hairs are in the telogen phase at any given time.

Exogen: This is your active shedding process, in which the fiber is dislodged from the scalp due to the new fiber growing out from under it. The average person loses about 50-100 strands of hair every day (and yes, that is completely normal).

To summarize, most of the hairs on your head (around 90%) are actively growing and will do so for anywhere from two to six years, at which point they will stop and begin to detach from the scalp. After a few months, the new hair growth from that follicle will push out the previous hair and continue the cycle! 

When the scalp is healthy and the growth cycle is working the way it should, this process is repeated about 20-30 times throughout a person’s lifetime. Many factors, however, from hormonal abnormalities to stress - even styling choices - can disrupt the cycle, causing increased hair fall, thinning, or even hair loss.