Many individuals observe an increase in hair shedding during warmer months, often noticing more strands in their brush or shower drain. Scientific understanding indicates a biological basis for this phenomenon, suggesting hair shedding can fluctuate with the seasons. It is a natural part of the hair growth cycle, responding to various internal and external cues throughout the year.
The Science of Hair Growth and Shedding
Human hair undergoes a continuous growth and shedding process, following a cycle with distinct phases. The anagen, or growth phase, is the longest, where hair follicles actively produce strands. This phase can last two to eight years, with 85% to 90% of scalp hairs typically in this active growth stage. Hair grows approximately 1 to 1.5 centimeters per month.
Following anagen is the catagen, or transitional phase, which is short, lasting two to three weeks. During catagen, hair growth slows, and the follicle shrinks and detaches from its blood supply. The final stage before shedding is the telogen, or resting phase, lasting two to four months.
In the telogen phase, hair is no longer actively growing but remains in the follicle until shed. The exogen phase is the actual shedding period, where old hairs are released for new growth. At any given time, 10% to 15% of scalp hairs are in the telogen phase. This continuous cycle means shedding a certain number of hairs daily is a normal physiological process, allowing for hair regeneration.
Seasonal Hair Changes in Humans
Humans experience seasonal hair shedding, often with a noticeable increase in late summer or early autumn. Studies indicate the proportion of hair follicles in the telogen (resting) phase peaks during summer months, leading to increased shedding a few months later. This pattern suggests hairs enter the resting phase during summer and are shed around August or September.
The exact biological mechanisms driving this seasonal pattern are not fully understood, but researchers propose several factors. Changes in daylight duration (photoperiodism) influence the hair growth cycle. Hormonal fluctuations, including melatonin, cortisol, estradiol, and follicle-stimulating hormone, may also play a role as they shift with changing seasons.
Environmental stressors in summer, like increased sun exposure and UV radiation, can impact hair health. UV rays weaken hair strands, making them prone to breakage and shedding. While humans do not shed hair for thermoregulation like many mammals, these environmental and physiological responses contribute to observed seasonal variations in hair shedding.
Distinguishing Normal Shedding from Excessive Hair Loss
Understanding the difference between normal hair shedding and excessive hair loss is important for assessing scalp health. On average, a person sheds between 50 and 100 hairs per day as a routine part of the hair growth cycle. This amount is normal and does not lead to noticeable thinning because new hair growth replaces the shed strands.
However, a sudden, significant increase in shed hair, such as large clumps in the shower or on your brush, might indicate excessive shedding. This condition, medically termed telogen effluvium, occurs when more hair follicles prematurely enter the resting phase and are shed. While seasonal changes can trigger this temporary condition, other factors like stress, illness, or certain medications can also cause it.
Signs that warrant further investigation include bald patches, a noticeable reduction in overall hair volume, or a receding hairline. While seasonal shedding is temporary and resolves on its own, persistent or unusually heavy hair loss could suggest an underlying issue. Consulting a healthcare professional, such as a dermatologist, can help determine the cause of concerning hair changes and provide appropriate guidance.