The hair growth cycle is the repeating sequence each follicle moves through: anagen (active growth lasting years), catagen (a brief transition) and telogen (rest and shedding). At any time most scalp hairs are in the growth phase, which is why daily shedding of 50 to 100 hairs is normal.
Most hair loss conditions are disruptions of this cycle: pattern loss shortens the growth phase over time, while telogen effluvium tips too many follicles into rest at once. Identifying which disruption is occurring guides the right treatment.