The hair follicle is a complex, miniature organ within the skin that operates in a precise, repeating sequence of stages rather than continuous growth. This organized, cyclical process ensures that hair is constantly renewed, preventing simultaneous shedding. The cycle moves each individual hair through periods of vigorous growth, controlled regression, and quiet rest before the process begins anew. Understanding these distinct phases provides insight into the biology of hair density, length, and the common causes of hair thinning or loss.
The Primary Hair Growth Phases
Hair growth starts with the anagen phase, the longest and most active period of the cycle. During this stage, cells at the base of the follicle, known as the matrix, divide rapidly to form the hair shaft through keratinization. This active production means the hair strand is firmly connected to the dermal papilla, which supplies the necessary blood and nutrients for growth. The duration of anagen largely determines the maximum length a hair can achieve, lasting between two to seven years on the scalp, but only a few months for body hair like eyebrows.
When the programmed growth period ends, the follicle enters the catagen phase, a short, transitional stage. This phase signals the cessation of active growth. Cellular activity stops, and the hair follicle shrinks dramatically to about one-sixth of its former length, detaching from the nourishing dermal papilla. This regression, which prepares the follicle for its resting state, lasts for approximately two to three weeks.
The Resting and Shedding Mechanisms
The Telogen Phase
Following the active regression of catagen, the follicle transitions into the telogen phase, a period of rest. During this time, the hair strand is fully keratinized and is referred to as a club hair, anchored in the now-dormant follicle. The telogen phase typically lasts for about three months, with roughly 10 to 15 percent of scalp hairs being in this state at any given moment. Although the hair is not growing, the follicle prepares for the start of the next cycle, positioning stem cells for renewed activity.
The Exogen Phase
The active release of the club hair occurs during the exogen phase, the distinct shedding stage. This process involves the physical detachment of the old hair shaft from the follicle, often facilitated by the emergence of a new anagen hair or routine actions like brushing and washing. Losing 50 to 100 hairs daily is considered normal physiological shedding as follicles move through this stage. In some cases, a prolonged period known as kenogen can occur, where the follicle remains empty after shedding, delaying the start of the new growth phase.
Factors That Regulate the Cycle
Hormones play a significant regulatory role in the timing and duration of the hair cycle. Androgens, such as dihydrotestosterone (DHT), are known to miniaturize hair follicles and shorten the anagen phase in genetically susceptible individuals, leading to pattern hair loss. Conversely, estrogen can extend the anagen phase, which is why many women experience thicker hair during pregnancy.
Genetic factors inherently determine the potential length of the anagen phase, establishing the maximum length a person’s hair can naturally grow. Nutritional status also profoundly impacts the cycle, as hair cells require a steady supply of resources like protein, iron, zinc, and B vitamins for rapid cell division. Deficiencies in these nutrients can prematurely force hairs into the resting phase, resulting in increased shedding.
Major physiological stress, whether due to severe illness, psychological trauma, or rapid weight loss, can trigger a condition called telogen effluvium. This event causes a sudden, synchronized shift where a large percentage of anagen hairs prematurely enter the telogen phase. The resulting noticeable increase in shedding occurs about two to four months after the stressful trigger.