The perception that hair growth slows down during the colder months is a common experience. Hair growth follows a steady rate, but its overall health and length are subject to many factors. This article examines the scientific evidence regarding seasonal changes in hair biology and explores the environmental reasons behind the winter slowdown many people observe. We will uncover whether the growth rate truly decreases or if other seasonal factors create the illusion of slower growth.
Debunking the Myth: Seasonal Changes in Growth Rate
The underlying rate at which hair grows from the scalp, measured in millimeters per day, remains consistent regardless of the season. On average, human hair grows about 0.3 to 0.4 millimeters daily, translating to approximately one centimeter per month year-round. However, the proportion of hairs actively growing fluctuates throughout the year, which is the source of this common misconception.
Scientific studies indicate that the hair cycle is not uniform across all seasons. Research has shown that the percentage of hair follicles in the active growth phase, known as Anagen, can peak around March. Conversely, a lower proportion of follicles may be in the Anagen phase later in the year, with an increase in hairs entering the resting phase.
This seasonal shift means that while a single strand of hair grows at its usual speed, a smaller percentage of total hairs are in a growth state during late summer. This leads to increased shedding in the fall and early winter months. The perceived slowing of growth is often a delayed effect of more hairs resting and shedding, rather than a true drop in the growth velocity of individual strands.
The Hair Growth Cycle Explained
To understand seasonal fluctuations, it is necessary to grasp the three distinct, cyclical phases of hair growth that every follicle undergoes.
Anagen (Active Growth)
The vast majority of the hair on the scalp is in the Anagen phase, which is the stage of active growth. During this period, cells at the base of the follicle rapidly divide to form the hair shaft. This phase can last anywhere from two to seven years, primarily determined by genetics.
Catagen (Transition)
Following the Anagen phase is the Catagen phase, a brief transitional stage that typically lasts only two to three weeks. In Catagen, hair growth slows dramatically, and the hair follicle shrinks and detaches from the blood supply. Only about one to two percent of the hair on the scalp is in this transition phase.
Telogen (Resting)
The final stage is the Telogen phase, the resting period before the hair is shed and the cycle begins anew. This phase lasts for two to four months, during which the hair is held loosely in the follicle. Approximately 10 to 15 percent of scalp hairs are in Telogen at any time, and the old hair is eventually released to make way for a new Anagen hair.
Why Hair Seems Slower in Winter
If the actual growth rate remains stable, the common feeling that hair grows slower in winter is largely due to environmental stressors and increased breakage.
Dehydration and Brittleness
Winter air holds less moisture, and indoor heating further lowers humidity, creating a dry environment. This lack of moisture causes the hair shaft to become dehydrated, making it stiff, brittle, and susceptible to snapping easily.
The harsh cold outdoors followed by dry heat indoors creates temperature shock, which can compromise the hair’s outer layer. This structural damage often results in breakage along the hair shaft. This is mistaken for slower growth because the hair is not reaching its full potential length, as the ends are continually breaking off.
Mechanical Stress
Mechanical stress is another significant contributor to perceived slow growth during the colder months. Winter accessories such as hats, scarves, and hoods create friction against the hair. This friction weakens the strands and leads to increased physical damage and breakage. This constant rubbing can accelerate the wear and tear on the hair, making it appear thinner and shorter.
Reduced Scalp Circulation
Cold temperatures can lead to vascular constriction, which temporarily reduces blood flow to the scalp. Since blood carries the nutrients necessary for follicle activity, reduced circulation can make it harder for the hair follicles to receive optimal nourishment. While this does not stop growth, it can affect the overall health and strength of the new hair being produced.