Stress can cause sudden hair loss by literally flipping the switch on the hair follicle’s growth stage from an active to a resting phase. Once the follicle enters this resting phase prematurely, it stays there for about three months, after which time a large amount of hair will be shed. Usually by then the person has recovered from the stressful event so she will regain a new head of hair.
Women who go through pregnancy and experience sudden hair loss about three months after childbirth can blame their hormones for that loss. Between 20 and 45 percent of mothers lose hair after giving birth as their estrogen and progesterone levels drop, and hair follicles are thrust into the inactive phase. Luckily, most mothers will regain their head of hair nine to twelve months after the child’s birth (assuming they get a good handle on their stress levels caused by a baby).
From The Mind-Beauty Connection: 9 Days to Less Stress, Gorgeous Skin, and a Whole New You by Amy Wechsler.