The hair growth cycle is made up of three phases: anagen, catagen and telogen.
The anagen phase is also referred to as the active phase of hair growth. The anagen phase lasts between 2-8 years, wherein hair grows at about 1 cm every 28 days. At least 90% of all hairs are in this active phase of growing at any given time.
This is followed by the catagen phase, which is a period of 2-4 weeks, where in the hair goes through a transitional phase. At this point, the existing hair is almost completely degraded. At any given time, at least 3% of all hair is in this phase.
The last phase, telogen, is also referred to as the resting phase. This phase lasts for about 2-4 months, and signals the start of hair shedding. During this time, hair is either pulled out (when combed) or naturally falls out on its own. As much as 50-100 hairs can be shed during this time, and at least 6-8% of all hairs can be in the telogen phase at any given time.
Many factors can disrupt the hair growth cycle and cause temporary hair loss. These factors include:
- Thyroid disease
- Hormonal and nutritional imbalances
- Certain skin diseases