Hair growth is cyclical. Throughout an individual’s life, the hairs all over their body are going through a long growth phase followed by a brief transition phase and then an intermediate resting phase before the specific hair is shed and a new one begins its long growth phase in the first one’s place. Growth, transition, rest, shed. Growth, transition, rest, shed. This pattern, which varies in exact duration from one person to the next and one body area to the next, is most frequently discussed in terms of its three main phases, the anagen phase, the catagen phase, and the telogen phase.

The anagen phase, or growth phase, can last anywhere from 2 to 6 years for scalp hairs. During this time, the follicle’s cells in the root are dividing, receiving nutrients that fuel growth from the bloodstream that are delivered via the papilla. Scalp hairs in the anagen phase grow on average about 6 inches per year. Hairs on other parts of the body, such as eyebrows or arm hairs, have a much shorter anagen phase, and thus, are shorter hairs than those on the scalp.

The catagen phase, or transition phase, is the shortest of the three, lasting only a few weeks for scalp hairs. During this time, the hair moves (or transitions) from active growth to a resting state. The mechanism through which this is achieved involves the root attaching to the shaft and detaching from the papilla, cutting the hair off from its source of nutrients.

The telogen phase, or resting phase, typically lasts about 2 to 3 months for scalp hairs. During this time, the non-growing, detached hair is referred to as a club hair. It is essentially dormant until it falls from the head. If it has not been shed by the time the next anagen phase begins, it is forced out by the newly growing hair. For those hairs on other parts of the body such as eyebrows or arm hairs, which, as discussed above, have a shorter anagen phase, the telogen phase is actually longer, leaving the short hairs at rest for more time than on they would be on the scalp.

Fortunately, not all hairs experience these phases at the same time. If they did, a person would experience years of growth followed by complete baldness. Instead, most hairs are in different stages than those surrounding them. At any given time, about 85-90% of hairs are in the anagen phase meaning only 10-15% are in the catagen and telogen phases, with more in the latter than the former. Some noticeable hair shedding occurs (up to 100 each day), but a healthy head of hair appears consistently full.

In less healthy manes, hair loss can sometimes be explained by a shift in the percentage of hair in the anagen and telogen phases. If follicles that would typically be in the growth phase enter the resting phase early, then the percentage of hairs in the anagen phase drops and that of those in the telogen phase increases, leading to a period of shedding a higher than typical amount of hair. This type of hair loss is referred to as telogen efflivium and can be caused by a number of things, including stress and subsequent changes in dietary habits, sleep, and health. As we wrote in our discussion on the impact of stress on hair loss in April:

“When external or emotional stressors lead to physiological stress, the body responds by essentially taking the attention it was giving the hairs in the anagen phase and reallocating it to other areas in need, thus pushing a larger number of follicles into the resting state. After a few months, these resting hairs begin to shed. While it is normal to lose telogen phase follicles, the abrupt loss of so many new resting hairs when stressors have caused telogen effluvium makes a regular cycle of loss and growth into an unbalanced and noticeable process of hair thinning.”

Gaining comprehension of how healthy hair growth works is an important part of understanding irregularities. With the knowledge of expected growth patterns and mechanisms, we can then identify specific points of abnormal development and explore their causes.