How does the body’s internal clock affect hair growth cycles?

Roots

The gentle unfolding of life’s rhythms holds sway over every aspect of our being, even the delicate strands that crown our heads. Just as the earth turns, guiding day into night, so too does an unseen clock within us govern the quiet processes of growth and renewal. This internal timepiece, often termed the circadian rhythm, extends its influence far beyond our sleep and wakefulness, reaching into the very heart of our hair follicles, dictating the ebb and flow of their activity. To truly understand our hair, particularly textured hair with its remarkable resilience and unique needs, requires listening to this subtle, yet powerful, biological orchestration.

At its core, hair growth is a cyclical phenomenon, a recurring performance within each individual follicle. These cycles are not random occurrences; rather, they adhere to a meticulously choreographed sequence. Every single hair follicle operates with its own internal rhythm, allowing for the continuous, asynchronous growth that prevents all our hair from shedding simultaneously. This inherent autonomy of each follicle, while influenced by the body’s broader signals, ensures a constant canopy of strands.

What are the Fundamental Phases of Hair Growth?

Hair growth unfolds in distinct stages, each serving a particular purpose in the life of a strand. These phases represent a journey from active growth to eventual release, preparing the follicle for its next cycle.

• Anagen ❉ This is the active growth phase, a period of rapid cellular division within the hair follicle, resulting in the lengthening of the hair shaft. For scalp hair, this phase can span several years, determining the ultimate length a strand can attain. The vibrancy and strength of hair are deeply connected to the health of the follicle during this active period.
• Catagen ❉ A brief, transitional phase, catagen marks the end of active growth. During this time, the hair follicle begins to shrink, detaching itself from the dermal papilla, the source of its nourishment. It lasts only a few weeks, signaling a pause in the hair’s outward journey.
• Telogen ❉ This is the resting phase, where the hair remains in the follicle, but no active growth occurs. It is a period of quiescence, a time for the follicle to regroup and prepare for a new cycle. Following this, the hair naturally sheds, making way for a new strand to begin its anagen journey. This resting period can last for several months.
• Exogen ❉ While often considered part of telogen, exogen is the specific phase of active shedding, where the old hair is released from the follicle. This is the moment we observe strands on our brushes or in the shower, a natural part of the hair’s life cycle.

The rhythm of these phases is not merely a localized event within the follicle; it is intimately connected to the larger symphony of the body’s internal clock. This connection helps regulate the cellular activity within the hair follicle, influencing when stem cells are activated or quieted, thereby governing the hair growth cycles.

The body’s internal clock orchestrates the intricate dance of hair growth, guiding each strand through its distinct phases of active lengthening, quiet transition, and eventual release.

How does the Body’s Circadian Clock Operate?

The body’s circadian clock, a marvel of biological timing, is an internal system that aligns our physiology and behavior with the approximately 24-hour light-dark cycle of our planet. This profound influence extends to nearly every cell and organ, establishing a predictable rhythm for various bodily functions. At its helm lies the suprachiasmatic nucleus, a central pacemaker in the brain that receives signals from light and temperature, calibrating the entire system.

Beyond this central conductor, peripheral clocks exist in various tissues, including the skin and hair follicles, operating with a degree of independence yet remaining synchronized with the master clock. These local oscillators integrate environmental cues and metabolic signals, fine-tuning their activities to local needs. For hair follicles, this means that while the central clock sets a broad tempo, the follicle itself possesses its own timekeeping machinery, adapting its cellular processes to daily and seasonal shifts.

The mechanism of this internal clock involves specific genes, often termed “clock genes,” such as BMAL1 and PER1. These genes operate in feedback loops, activating and inhibiting each other in a rhythmic fashion, creating the approximately 24-hour cycle. Their protein products regulate the expression of other genes, influencing a cascade of cellular activities, including cell division, metabolism, and even DNA repair. In the context of hair, these clock genes play a direct role in regulating the cell cycle progression of hair follicle cells, influencing when they divide and differentiate.

Ritual

Our daily routines, from the moment we greet the dawn to the quiet descent into slumber, carry a subtle resonance with the body’s inner timings. Understanding how these rhythms influence our hair growth is not simply an academic pursuit; it is an invitation to align our care rituals with our body’s natural cadence, nurturing our strands with deeper wisdom. The interplay between our internal clock and the hair’s vitality is a practical consideration, guiding us toward practices that honor our inherent biological rhythms.

How do Daily Rhythms Impact Hair Follicle Activity?

The cells within our hair follicles, like many other cells in the body, exhibit daily rhythms in their activity. This means that certain processes, such as cell division and gene expression, are more active at particular times of the day. Research indicates that the hair matrix cells, which are responsible for hair growth, may divide faster in the morning than in the evening. This daily mitotic rhythm, a direct consequence of the peripheral circadian clock within the hair follicle, suggests that even subtle shifts in our daily patterns can have an effect on the cellular machinery of hair growth.

Consider the impact of sleep, a fundamental pillar of our circadian rhythm. When sleep is consistently disrupted or insufficient, the body’s stress hormones, like cortisol, tend to rise. Elevated cortisol levels can push hair follicles prematurely into the resting phase, leading to increased shedding. This connection underscores the importance of restorative sleep, not just for overall well-being, but as a silent ally in maintaining a robust hair growth cycle.

The body’s repair and regeneration processes intensify during deep sleep. This includes increased blood flow to the scalp, delivering vital oxygen and nutrients to hair follicles. Hormones that support hair growth, such as melatonin, are also balanced during this time. When these processes are disturbed by poor sleep, hair follicles may become weakened, and regrowth can slow.

Aligning daily practices with our circadian rhythm, especially through consistent, restorative sleep, provides a gentle yet powerful foundation for robust hair growth.

Can External Factors Influence Hair Growth Cycles?

Beyond the internal clock, external factors, particularly light exposure, play a significant role in synchronizing our circadian rhythms and, by extension, affecting hair growth. The amount and type of light we encounter throughout the day signal to our central clock, which then communicates with peripheral tissues like hair follicles.

Seasonal variations in daylight, for example, have been linked to changes in hair growth patterns. A 2009 study involving over 800 healthy women found that hair growth and loss patterns exhibit seasonal behaviors. The study observed a maximal proportion of hair in the telogen (resting) phase during the summer, with a second, less pronounced peak in spring.

Conversely, telogen rates were lowest in late winter. This suggests a natural rhythm of increased shedding during warmer months, potentially as an adaptation to environmental conditions.

The advent of modern living, with its constant exposure to artificial light, presents a contemporary challenge to these ancient rhythms. Blue light, emitted from electronic screens, can disrupt the body’s natural sleep-wake cycle, particularly when viewed at night. This disruption can interfere with the regenerative sleep phase and circadian rhythm, potentially contributing to conditions such as telogen effluvium, a stress-related hair disorder that results in increased shedding. The implications for hair health extend beyond just sleep; the body’s metabolic state can also be impacted, further influencing follicular vitality.

Consider the case of shift workers, whose schedules often clash with natural light-dark cycles. Their irregular sleep-wake patterns can lead to a disruption in melatonin secretion, a hormone vital for regulating circadian rhythms and also possessing receptors in hair follicles that may signal for hair growth. This constant state of biological misalignment can place the body under considerable stress, potentially affecting metabolic processes and, for some, contributing to noticeable hair thinning or loss.

To nurture hair with intention, we might consider the subtle shifts in our environment and daily practices. Dimming lights in the evening, establishing a consistent sleep schedule, and allowing our bodies to respond to the natural light of the day are gentle acts of alignment that support the hair’s inherent wisdom.

Relay

Beyond the visible rhythms of our days and nights, a profound dialogue occurs at the cellular level, where the body’s internal clock relays precise instructions to the very engines of hair growth. This deeper exploration uncovers the molecular mechanisms that connect chronobiology to follicular vitality, revealing a sophisticated interplay of genes, hormones, and cellular processes. It is here that we begin to appreciate the intricate dance that dictates not only when hair grows, but how robustly it thrives.

How do Clock Genes Regulate Hair Follicle Cycles?

The core of the circadian system lies in a set of specialized clock genes, such as CLOCK, BMAL1, PER1, and CRY1. These genes, found in virtually every cell, including those of the hair follicle, orchestrate rhythmic gene expression over approximately 24-hour periods. Their activity is not merely confined to setting daily cycles; emerging evidence indicates these same clock genes significantly influence the hair growth cycle, a process with a much longer periodicity, spanning weeks to years.

For instance, studies involving mice with mutations in core clock genes like Clock and Bmal1 have shown significant delays in the progression of the anagen (growth) phase. This suggests that these genes are not just observers of the hair cycle; they are active participants, modulating the timing and progression of hair growth. The hair follicle itself contains distinct compartments, such as the secondary hair germ and the dermal papilla, which exhibit prominent rhythmic circadian gene expression, particularly during the transition from telogen (resting) to early anagen.

The connection between the circadian clock and the hair cycle extends to the regulation of cell division. Clock genes influence the expression of key cell cycle control genes. In mice lacking BMAL1, for example, keratinocytes in a critical compartment of the hair follicles can become halted in the G1 phase of the cell cycle, impeding growth. This highlights a direct molecular link ❉ the internal clock, through its genetic machinery, can pause or promote the very proliferation needed for hair to grow.

The intricate choreography of hair growth cycles finds its deep roots in the rhythmic directives of our internal clock genes, shaping cellular division and follicular vitality.

What is the Role of Hormones in Circadian Hair Growth?

Hormones act as powerful messengers, relaying signals from the central circadian clock to peripheral tissues, including hair follicles. Melatonin, often associated with sleep regulation, serves as a prime example. Secreted by the pineal gland in response to darkness, melatonin not only helps synchronize our sleep-wake cycles but also directly influences hair follicle activity.

Receptors for melatonin have been identified in the dermal papilla of hair follicles, suggesting a direct role in signaling pathways that affect hair growth. Melatonin can influence the cyclic activity of hair follicles and has been shown to play a role in synchronizing anagen, catagen, and telogen phases, particularly in animals with seasonal molting.

Thyroid hormones, such as Thyroxine (T4), also stand as significant regulators of the hair growth cycle. They can prolong the anagen phase in hair follicles and influence the clock activity within these mini-organs. Individuals with thyroid dysfunction often exhibit a disordered circadian clock, which can, in turn, manifest as variations in hair growth. This connection underscores how systemic hormonal balance, guided by circadian rhythms, directly impacts the local environment of the hair follicle.

Beyond individual hormones, the broader hormonal milieu, influenced by the circadian system, affects hair health. For instance, the stress hormone cortisol, which exhibits a diurnal rhythm (typically higher in the morning and lower at night), can significantly impact hair growth when its rhythm is disrupted. Chronic elevation of cortisol, often seen with sleep deprivation or prolonged stress, can prematurely push hair follicles into the resting phase, leading to excessive shedding. This intricate web of hormonal communication, orchestrated by the internal clock, provides a comprehensive understanding of hair’s responsiveness to our body’s deeper states.

Can Hair Follicles Serve as a Window into Our Internal Clock?

The presence of functional circadian oscillators within hair follicles has opened a fascinating avenue for understanding human chronobiology. Because hair follicle cells express core clock genes in a rhythmic fashion, they can serve as a relatively non-invasive model for monitoring an individual’s internal circadian time. This has significant implications for various fields, from personalized medicine to understanding the health impacts of shift work.

Consider a study that aimed to develop a model for estimating circadian time using hair follicle cells. Researchers collected hair follicle cells from healthy subjects at different times of the day and analyzed the expression patterns of various circadian genes. They found that mRNA levels of genes like PER1, PER3, and NR1D1 oscillated in a circadian fashion with consistent phases across individuals. This research suggests that a single time-point sampling of hair follicle cells could potentially provide a reliable estimate of an individual’s endogenous circadian time, with one model demonstrating an error range of approximately 3.24 hours between actual and predicted time.

This capability is particularly compelling when considering populations whose circadian rhythms are frequently challenged, such as rotating shift workers. For these individuals, a serious time lag can develop between their internal circadian gene expression rhythms and their external lifestyle. The ability to assess this misalignment through a simple hair follicle biopsy offers a powerful tool for diagnosing circadian rhythm disorders and potentially tailoring interventions, including chronotherapy, to align treatments with an individual’s unique biological timing.

Such insights are not merely theoretical. They hold practical promise for understanding and mitigating the impact of modern lifestyles on our overall health, including hair vitality. By listening to the subtle messages of our hair follicles, we gain a deeper appreciation for the profound connection between our internal clock and the visible expression of our well-being.

Reflection

The quiet pulse of our internal clock, a rhythm that shapes our very being, casts a subtle yet undeniable influence over the life of our hair. From the unseen cellular choreography deep within each follicle to the seasonal shifts that touch our strands, the connection between our chronobiology and hair growth is a profound testament to the body’s inherent wisdom. This understanding invites us to view our hair not as an isolated feature, but as a responsive part of a larger, interconnected system.

By honoring these natural timings, whether through mindful sleep or an appreciation for the cycles of light and dark, we begin to align with the deeper flow that nourishes our hair from within. The journey of healthy hair, then, becomes a gentle partnership with our own unique biological cadence, a celebration of harmony between inner rhythms and outer radiance.

References

• Wong, S. & Sivamani, R. (2017). How Does the Circadian Rhythm Affect Hair Growth?. LearnSkin.
• Lin, K. K. & Lin, C. M. (2013). Local circadian clock gates cell cycle progression of transient amplifying cells during regenerative hair cycling. Proceedings of the National Academy of Sciences, 110(21), 8613-8618.
• Geyfman, M. & Andersen, B. (2010). Clock genes, hair growth and aging. Aging (Albany NY), 2(3), 105.
• Niu, Y. Wang, Y. Chen, H. & Liu, J. (2023). Overview of the Circadian Clock in the Hair Follicle Cycle. Biomolecules, 13(7), 1068.
• Niu, Y. Wang, Y. Chen, H. & Liu, J. (2023). Hair Follicles as a Critical Model for Monitoring the Circadian Clock. Biomolecules, 13(2), 226.
• Kunz, M. Seifert, B. & Trüeb, R. M. (2009). Seasonality of hair shedding in healthy women complaining of hair loss. Dermatology, 219(2), 105-110.
• Hsiang, E. Y. Semenov, Y. R. Aguh, C. & Kwatra, S. G. (2017). Seasonality of hair loss ❉ a time series analysis of Google Trends data 2004 to 2016. British Journal of Dermatology, 178(4), 1152-1155.
• Kobets, K. & Henry, M. (2025). Shedding Light on Hair Growth ❉ Can Red Light Therapy Really Restore Your Strands?. Byrdie.
• Kapoor, R. (2023). Not just the skin, increased screen time can also affect your hair — here’s how. The Indian Express.
• Schernhammer, E. (2016). The night shift killed my memory, mood, and hair. National Post.
• Oh, Y. J. Kim, S. J. & Kim, Y. C. (2020). Development of model based on clock gene expression of human hair follicle cells to estimate circadian time. Chronobiology International, 37(7), 1053-1061.