Hair growth cycle

Each scalp follicle cycles through four phases independently of its neighbours. Asynchrony is normal: at any moment, around 80 to 90% of follicles are in anagen, a small fraction in catagen, and 5 to 15% in telogen.

Anagen (growth) lasts 2 to 7 years. Anagen length determines maximum hair length. Most active hair production happens here.

Catagen (transition) lasts 2 to 3 weeks. The follicle stops producing hair and begins to retract.

Telogen (rest) lasts 2 to 4 months. The hair sits in place, not growing, before being released.

Exogen is the active shedding of the rested hair as a new anagen hair pushes up underneath.

In telogen effluvium, a stressor pushes a large fraction of follicles synchronously into telogen, and they all shed together a few months later. In pattern hair loss, anagen is progressively shortened over years (miniaturisation), so each cycle produces a thinner, shorter hair.