Hairs grow in a cycle with periods of growth (Anagen) than can last a number of years. When hairs reach the end the growing phase they regress (Catagen) and then rest for several months (Telogen) before being shed (Exogen). We lose 50-100 hairs per day in health, with 90% of all scalp hairs being in the growing phase and 10% being in either Catagen or Telogen. Each individual hair has it's own cycle. There are a number of factors that can cause hairs to become synchronised and to stop growing at the same time. After several months these hairs will all be shed at the same time, leading to more noticeable hair loss. Causes can include severe illness or psychological distress, marked weight loss, nutritional deficiency or medications.