The average human scalp has about 100,000 hairs, and each hair follicle grows in a repetitive cycle, which includes three phases:
Anagen – A growth phase, which typically continues for 150 weeks or more, depending on how long the hair can be grown on the scalp.
Catagen – A transitional phase, during which time, the bulbar portion of the follicle is almost completely degraded through apoptosis. Catagen lasts for 1 week.
Telogen – A resting phase that lasts for 12 weeks on the scalp. Approximately 50–100 telogen hairs are shed daily, mostly because of normal washing and combing.
Telogen effluvium occurs when factors such as extreme stress or hormonal imbalances cause the hair to skip the Anagen (growing) phase and enter the resting (telogen) phase. This causes a significant amount of hair to fall out about 2–4 months later. This type of hair loss is widespread over the scalp, so the decrease in volume may not be easily noticeable at first, and no bald spots will typically develop. If the conditions of extreme stress continue, the thinning will become apparent. Generally, when conditions improve, hair will start to re-grow within 6 months, and no medical treatment is necessary. If you’re unsure about the reason, your doctor can check for the underlying cause.