Your hair is not always growing. Instead, it goes through phases (periods or stages) of growth and phases of rest. In other words, your hair is either growing or it is resting. When the follicle has finished growing, it enters the resting phase. During the resting phase, the hair shaft breaks and the old hair falls out. The follicle then enters the growth phase once again. This repeats in a cycle, as new hair constantly replaces the old hair.