When the hair cycle functions normally, we are kept in a constant state of rest and regrowth; with 80% of our hair in the growing phase and a measly 20% kept in a dormant phase without growth taking place.
When there is a disturbance or a shock to our system, much of the hair on our heads can cease growing and enter into the dormant stage. This happens naturally which leads to large amounts of hair loss.
The change in the hair cycle and falling out of the hair is known scientifically as Telogen Effluvium, a condition found mostly among those who have under gone some physical change such as a trauma, a diet (crash diets specifically) or are at the moment undergoing some form of psychological malaise.