Hair growth cycle

Each hair follicle cycles through phases. Anagen is the growing phase, which on the scalp usually lasts several years. Catagen is a short transition of a few weeks. Telogen is the resting phase, which lasts around three months, after which the old hair is shed and a new one begins to grow.

Follicles cycle independently, so at any time most scalp hairs are growing and a smaller number are resting. That is why losing some hair every day is normal. Shedding in the region of 50 to 100 hairs a day is commonly cited as typical, although it varies from person to person and from day to day.

Many types of hair loss can be understood as changes to this cycle. In telogen effluvium, more hairs than usual enter the resting phase at once. In androgenetic alopecia, the growing phase gradually shortens and hairs become finer.