Hair Growth Cycle

Scalp hair rotates through distinct phases that determine length, shedding, and density.

Phases

Anagen (growth): lasts ~2–8 years on the scalp; 80–90% of hairs are in anagen at any time.
Catagen (transition): ~2–3 weeks; follicle shrinks and detaches from the dermal papilla.
Telogen (rest): ~2–4 months; the club hair is retained before shedding.
Exogen: release of the old club hair.
Kenogen: an empty interval before a new anagen begins (not always present).

Regulation and variation:

Hair-cycle timing is controlled by local signalling (Wnt, BMP, FGF, EGF, Hedgehog pathways) and dermal papilla cues; transcription factors such as DLX3 are crucial for differentiation and cycle control.
Cycle lengths differ by body site (scalp anagen years-long; eyebrows months-long) which explains why scalp hair can grow long while other regions remain short.
Up to 85% follicles may be in anagen on a healthy scalp, with ~10–14% in telogen and a small % in catagen; these proportions vary with age, hormones and medical state.

Average growth is ~1–1.5 cm per month, varying by age, genetics, health, and hair fibre thickness.

Hormones (thyroid, oestrogen/androgen balance), pregnancy/postpartum shifts, nutritional status, illness, and medications can shorten anagen or shift more hairs to telogen.

Telogen effluvium and clinical context:

Telogen effluvium is a common, usually reversible cause of diffuse shedding triggered by stressors (postpartum, febrile illness, surgery, rapid weight loss, severe psychological stress, some drugs). Hair shedding typically becomes noticeable 2–3 months after the trigger as follicles synchronise into telogen.
Chronic medical conditions (thyroid disease, iron deficiency, autoimmune disease) and some medications (antiepileptics, retinoids, anticoagulants) can alter cycle dynamics and provoke shedding.

Use cycle shifts to signal life changes (postpartum shed, stress‑related fall) and recovery arcs.
Distinguish shedding (more hairs in the brush) from breakage (short, snapped strands).

"Two months after the breakup, the plughole told the story first—a quiet drift of strands that slowed only when sleep and meals returned to normal."