Many mammals shed their fur or hair seasonally to adapt to temperature changes. This raises the question: do humans experience similar seasonal hair shedding? While human hair doesn’t serve the same thermoregulatory purpose, understanding our hair’s natural cycles can clarify if environmental shifts influence hair loss.
The Science of Human Hair Growth
Human hair growth follows a continuous cycle, with each strand undergoing distinct phases. The first is the anagen phase, the active growth period where hair follicles produce new hair cells. This phase is the longest, typically lasting between two and eight years, and determines the ultimate length hair can achieve. During this time, hair grows approximately 1 to 1.5 centimeters per month.
Following the anagen phase is the catagen phase, a short transitional stage lasting about two to three weeks. In this period, hair growth slows, and the hair follicle detaches from its blood supply, beginning to shrink.
The final stage is the telogen phase, a resting period that usually lasts for two to four months. During this time, the hair remains in the follicle but does not grow, preparing for shedding. The exogen phase is the shedding phase, where the old hair is released from the follicle, making way for new hair to begin its anagen phase. Losing between 50 and 100 hairs daily is considered normal, as different hairs are constantly in various stages of this cycle across the scalp. This natural turnover ensures a continuous renewal of hair.
Is Human Hair Loss Seasonal?
Many people report experiencing increased hair shedding during certain times of the year, often anecdotally noting more hair loss in autumn. Scientific studies show human hair growth and shedding can exhibit seasonal patterns. Research indicates a higher proportion of hair follicles enter the resting, or telogen, phase during summer months. This higher percentage of resting hairs typically leads to increased shedding a few months later, aligning with observations of more noticeable hair loss in the fall.
A study found an annual periodicity in hair shedding, with a peak proportion of telogen hairs occurring in summer. This suggests that the hair cycle can synchronize to some extent, leading to more hairs being shed simultaneously. Another analysis, using Google Trends data, supported this idea, showing that searches for hair loss tend to peak in the summer and fall across various countries.
While autumn shedding is frequently reported, some individuals may notice increased hair loss in other seasons. Spring can also see a rise in shedding for some, potentially linked to an increase in temperature and sunlight influencing hormonal balances. These seasonal variations are often linked to telogen effluvium, where a larger number of hair follicles prematurely enter the resting and shedding phases due to various triggers.
Why Human Hair Shedding Might Vary
The observed variations in human hair shedding throughout the year can be influenced by several biological and environmental factors. Hormonal fluctuations play a role; changes in daylight exposure can affect melatonin production, which regulates the hair growth cycle. A decrease in melatonin, often associated with longer daylight hours in summer, might lead more hair follicles to transition into the shedding phase.
Temperature changes also contribute to variations in hair shedding. Hot summer weather, particularly with increased sun exposure, may stress the scalp, potentially triggering more follicles to enter the telogen phase. Conversely, the dry indoor heating and colder temperatures of winter can strip moisture from the scalp and hair, making strands more brittle and prone to breakage.
General health factors further contribute to hair shedding patterns. Stress, for example, can induce telogen effluvium, causing a temporary increase in hair loss. Significant life events or ongoing stress can disrupt the hair cycle, leading to noticeable shedding a few months later. Nutritional status also impacts hair health, as deficiencies in certain vitamins or minerals can affect the hair growth cycle and overall hair density.
When to Consult a Professional
While some degree of seasonal hair shedding can be a normal physiological response, certain signs indicate professional medical advice may be beneficial. If you experience sudden, excessive hair loss that goes beyond your usual shedding, or if you notice distinct bald patches, seek professional advice. A daily loss of more than 100 to 150 hairs consistently could signal an underlying issue.
Other concerning symptoms include noticeable overall thinning of your hair, particularly if it progresses rapidly. Scalp irritation, redness, scaling, or itching accompanying hair loss should also prompt a visit to a doctor or dermatologist. These could be signs of a medical condition affecting the scalp or hair follicles. A healthcare provider can help distinguish between typical shedding and hair loss conditions, some of which may require specific treatments.