Research confirms that a discernible pattern of seasonal hair shedding exists. This phenomenon, sometimes referred to as seasonal effluvium, represents a natural, evolutionary remnant in human biology. While the question asks specifically about winter, increased shedding is commonly observed during transitional periods of the year. This temporary increase in hair fall is linked to changes in the environment. The body’s subtle response to seasonal shifts causes a mild, synchronous increase in the number of hairs being shed.
Understanding the Human Hair Growth Cycle
Hair growth follows a cycle across the scalp, ensuring that not all hairs are shed at once. This cycle is divided into three primary phases. The initial and longest stage is the Anagen phase, the active growth period, typically lasting from two to seven years and involving about 85% to 90% of all scalp hairs at any given time.
Following this growth, the hair follicle enters the short Catagen phase, a transitional period lasting approximately ten days to three weeks. During this time, hair growth ceases, and the lower part of the follicle begins to shrink and detach from the blood supply. The final stage is the Telogen phase, a resting period that usually lasts about three months. This is when the old hair is held in place before being released during the Exogen phase, making way for a new Anagen hair to grow from the same follicle.
The Biological Basis of Seasonal Shedding
The underlying mechanism for seasonal hair fall is believed to be an evolutionary adaptation tied to environmental cues. Hair follicles are signaled to retain more hair during the summer months to provide greater protection for the scalp against intense ultraviolet radiation. This increased retention of hair during the summer then leads to a delayed, synchronized shedding event later in the year.
The main trigger for this biological shift appears to be the change in daylight hours, known as the photoperiod. Alterations in the amount of light entering the eye can impact the production of hormones such as melatonin and prolactin, which influence hair follicle activity. These hormonal changes signal a higher percentage of hair follicles to prematurely shift from the Anagen (growth) phase into the Telogen (resting) phase.
Timing the Peak: Why Shedding Appears in Colder Months
While the biological trigger often occurs during the longer daylight hours of late spring or summer, the physical shedding is delayed due to the nature of the hair cycle. The hair follicle requires a resting period once it enters the Telogen phase before the hair strand is released. This dormant Telogen phase lasts for two to three months.
Consequently, the increased number of hairs that were signaled to enter the resting phase in late summer or early fall will not physically fall out until approximately 90 to 100 days later. This biological lag time is why the peak of noticeable hair shedding is most frequently observed during the autumn and early winter months. The hair that was protected during the sunniest part of the year is now shed as the body adjusts to the shorter days and colder temperatures.
Differentiating Seasonal Loss from Other Hair Thinning
It is normal for humans to shed between 50 and 100 hairs every day. During seasonal peaks, this daily amount may temporarily increase, but the shedding should be diffuse across the scalp and last for a finite period, usually six to twelve weeks. The key difference between seasonal loss and pathological hair conditions lies in the quantity, duration, and pattern of the loss.
If the shedding exceeds the normal range for an extended period, or if it results in noticeable changes in scalp visibility or density, it may indicate a more serious condition. Conditions like chronic telogen effluvium or pattern baldness (androgenetic alopecia) typically present as persistent thinning, a receding hairline, or patchy loss, rather than a temporary, diffuse increase in shed hairs. If extreme volume loss, bald patches, or if the excessive shedding lasts longer than three months, it is advisable to consult a healthcare professional for an accurate diagnosis.