Catagen is the transitional phase in the hair growth cycle that follows the active growth stage, known as anagen. During catagen, which lasts about two to three weeks, hair follicles shrink and detach from the blood supply, signaling the end of hair growth. This phase is crucial as it prepares the hair for the next stage, called telogen, where the hair eventually falls out. Approximately 3% of hair is in the catagen phase at any given time, making it a brief but essential part of the hair's life cycle.