A stressful event, such as a severe illness, childbirth, or emotional trauma, can be a shock to the system. That can result in telogen effluvium, a condition in which about 30% of hairs enter the resting phase and fall out a few months later.
Up to 90% of someone’s hair is typically in the growing phase at any giving time, while the rest are in the telogen (or resting) phase. Hairs in the telogen phase fall out after a few months, which explains the strands we usually find in our brushes and bathtub drains.