Hair does not grow continuously and does not fall out at random. Each follicle runs a repeating cycle with distinct phases, and almost everything people notice about their hair follows from the timing of that cycle.
Three phases, running out of step
The growth phase, anagen, is when the follicle actively produces hair fibre. On the scalp it lasts a period measured in years, which is why scalp hair reaches a length that eyebrow hair never does.
A brief transition phase follows, during which the follicle detaches from its blood supply and stops producing fibre.
Then comes a resting phase of some months, at the end of which the old hair is released and a new growth phase begins beneath it.
Maximum length is set by phase duration
Hair grows at a fairly steady rate, so the length it can reach is determined by how long the growth phase lasts before it ends.
Growth phase duration differs by body site, which is why body hair stops at a short length without ever being cut.
It differs between individuals too, and that difference sets the practical ceiling on how long someone's hair can grow regardless of care.
Shedding is a consequence of asynchrony
Human follicles cycle independently of one another rather than in waves, so at any moment the scalp holds follicles in all three phases at once.
A modest fraction is resting at any given time, and hairs released from those follicles are what appear on a brush or in a drain.
Daily shedding of some tens of hairs is therefore the expected output of a system working normally, not evidence of loss.
A shock shows up months later
A significant physical stress, such as a severe illness, surgery, childbirth or a period of poor nutrition, can push many follicles into the resting phase simultaneously.
Because the resting phase runs for months before release, the shedding appears well after the event that caused it, and the connection is often missed.
The follicles are generally not destroyed in this pattern, and cycling usually resumes, but the delay works in both directions and recovery is equally slow to appear.
Why any change takes a season to judge
Anything that acts on hair acts on the follicle, and the follicle can only express that change through a new growth phase.
Visible results consequently lag by months, which is the timescale on which dermatologists assess whether anything has worked.
Sudden heavy shedding, patchy loss, or loss with scalp scaling or scarring are different presentations and should be examined rather than waited out.