Telogen is the resting stage of the hair growth cycle, opposite of anagen phase. Approximately 10-15% of all hairs are in the telogen at any given time. This phase usually lasts 2-4 months. Telogen effluvium is a condition that can be caused by a number of things, including childbirth, major surgery, severe illness, or extreme stress. It can cause a large number of hairs to enter the phase at once, leading to loss of hair.