Article updated and reviewed by Michael S. Lehrer, MD, Clinical Assistant Professor, Department of Dermatology, University of Pennsylvania on April 18, 2005.

Each hair grows in cycles—it grows, rests, and then falls out. Usually, this cycle repeats approximately yearly. At any time, about ninety percent of a person's scalp hair is growing, a phase that lasts between two and six years. Ten percent of the scalp hair is in a resting phase that lasts between two and three months. At the end of its...