While humans do not undergo the dramatic, synchronous molting observed in many other mammals, hair turnover, or shedding, is a constant biological function. A person typically loses between 50 and 100 hairs daily as a normal part of this renewal process. The observation of increased, temporary hair fall during certain times of the year prompts the question of seasonal shedding. Scientific investigation confirms that subtle, measurable seasonal variations influence the timing of our hair cycle.
The Standard Human Hair Growth Cycle
The growth and shedding of human hair follow a precise, continuous cycle unique to each individual follicle. This cycle is generally divided into three distinct phases: anagen, catagen, and telogen. Human hair follicles operate independently, which prevents the mass hair loss seen in animals that shed their entire coat at once.
The anagen phase is the active growth period, the longest stage of the cycle, lasting anywhere from two to eight years for scalp hair. About 80 to 90 percent of scalp hairs are in this active state, where cells in the hair root divide rapidly to form the hair shaft. Hair grows approximately one centimeter per month during this phase.
Following anagen is the catagen phase, a short transitional period lasting only about ten days to three weeks. In this phase, the hair follicle shrinks and detaches from the dermal papilla, which supplies it with nutrients. Only about one percent of hairs are in this regressive state at any moment.
The cycle concludes with the telogen phase, a resting period that typically lasts for two to four months. During this time, the old hair is fully formed but inactive, remaining anchored in the follicle before it is released. This release marks the beginning of the exogen phase, or shedding, as the follicle prepares to re-enter the active anagen phase.
Scientific Evidence Linking Shedding to Seasonality
Dermatological research has consistently demonstrated that the human hair cycle exhibits an annual periodicity. Studies tracking the proportion of hair follicles in the telogen phase across populations show a clear pattern that correlates with the seasons. This fluctuation highlights a subtle, evolutionary remnant of seasonal shedding.
The most consistent finding across multiple studies is a peak in the percentage of hairs entering the telogen phase during the summer months. Research conducted in the Northern Hemisphere finds that the maximal proportion of hairs in the resting stage often occurs between July and September. This timing is significant because the telogen phase lasts approximately three months before the hair is shed.
A surge in hair follicles entering the resting phase in late summer typically results in a noticeable increase in hair shedding during the autumn season. One study confirmed this annual periodicity, showing that telogen rates were highest in summer and lowest during late winter.
Environmental and Hormonal Influences on Hair Timing
The biological mechanism driving this seasonal fluctuation is thought to be a response to changing external cues, primarily sunlight and temperature. One prevailing theory suggests that increased exposure to ultraviolet (UV) radiation during the summer months acts as a trigger. This UV exposure may prompt hair follicles to prematurely enter the telogen phase as a protective measure to shield the scalp from sun damage.
Temperature fluctuations also play a role, as the body attempts to adapt to warmer and cooler conditions. The environmental shifts are believed to modulate the hair cycle via subtle changes in hormone levels that vary seasonally.
These hormones include Melatonin, which is regulated by light exposure and is known to influence the hair cycle by promoting the anagen phase. Melatonin levels are naturally lower during periods of increased daylight, which may contribute to the summer increase in hairs entering the telogen phase.
Other hormones, such as prolactin and certain sex steroids like estradiol, also exhibit seasonal variations and are hypothesized to subtly affect follicle activity. The cumulative effect of these hormonal shifts orchestrates the temporary, yet measurable, increase in hair shedding observed predominantly in the autumn.