How Does the Circadian Rhythm Affect Hair Growth?

​Hair has natural resting and growth cycles

Hair Growth Cycle

Hair growth is a vital part of our lives and scalp, facial, and body hairs are some of the few body parts that one can change and manipulate. In addition to piercing and tattooing skin, hair grooming allows an individual to uniquely present themselves. Mature hair undergoes several phases in the hair growth cycle including anagen, catagen, telogen, and exogen.[1] Anagen is the phase of active growth. During this time, factors outside of the body may affect hair growth. For instance, hair usually grows faster in the summer months as compared to the winter. The catagen phase begins when anagen ends. It is a shorter transitional phase where hair stops growing. Telogen is a resting phase that transitions into the exogen phase where old hairs are shed.[2] After the hair falls out, the follicle is inactive for some time before the hair’s biological clock restarts. Each hair follicle is independent from the others and undergoes its own hair growth cycle at different times from each other. This is what prevents hair from falling out all at once.

Table 1.  Hair Growth Phases

Hair Growth Phase



Growth phase


Transition from growth phase to resting phase


Resting phase


Phase where resting hair is shed from the body


Circadian Rhythm and Circadian Genes

The circadian rhythm is described as a biological clock that affects how the body functions over a 24-hour cycle. The circadian rhythm is affected by the environment, such as light and darkness. This biological clock is regularly matched to a 24 hour day through exposures to light and dark.[3] The circadian clock is centered in parts of our brain that can receive signals from light and temperature.[4] Clock genes are genes that help contribute to our body’s circadian rhythm, also known as the molecular gears of circadian clocks.[5] Therefore, for our circadian clock to be able to respond to the environment, our clock genes also rely on both the internal biology of the body as well as factors outside of the body that work together to affect the hair’s biological clock.


The Hair Growth Biological Clock

There is evidence that the circadian clock, which adjusts the body’s biology to the day’s natural cycle, regulates skin functions. For each hair growth cycle, hair follicles make new hair. The ability to regenerate hairs throughout life is from the hair stem cells in the hair follicle and the influence of the circadian rhythm.

Once growth is complete during the anagen phase, clock genes are expressed in the hair cells at the bottom of the hair follicle. Thus, the circadian clock differs among the phases of the hair growth cycle.[2] The circadian clock can adjust how the hair stem cells are turned off or on, which controls the hair growth cycles. For instance, through a clock gene called BMAL1, stem cell genes can be affected to become more or less active. If mutations arise in clock genes, such as deleting BMAL1, stem cells may become inactive leading to poor hair growth.[6]


Disturbing the Hair’s Biological Clock

Studies have shown that disrupting the circadian clock can prolong the phase of active hair growth. BMAL1 and PER1 are core clock genes in the human hair cycle.[7] Because the circadian clock influences cell activity at different stages of the hair cycle, clock gene mutations can result in subtle hair cycle variations.[8] These clock gene dysfunctions may cause certain disorders.[7] The BMAL1 and PER1 clock genes produce signals that terminate the growth phase, anagen. Silencing both of these genes can prolong the hair growth cycle and lead to longer hair.

While disrupting clock genes (BMAL1 and PER1) and prolonging the anagen phase of active hair growth supports the idea that the hair follicles exhibit a biological clock based activity, other hormones are important too. For example, thyroid hormones also prolong anagen in hair follicles.[9] Thyroid hormones are one of the main regulators of the hair growth cycle. Thyroxine (T4) also regulates clock activity of hair follicles and patients with thyroid dysfunction may also show a disordered circadian clock.[9]


Sleep and its Effects on Hair Growth

Some sleep disorders are related to our circadian rhythms. In some sleep disorder, sleep occurs abnormally early or late. Studies have shown that these disorders often correlate to mutations in clock genes that then lead to an irregular biological clock.[10] On the other hand, sleep deprivation could alter clock gene expression. The mechanism of this is due to the decreased binding of certain clock genes that have been associated with lack of sleep.[11] Our sleep and wake cycles are then affected so that we may not be as alert as we would like to be in the morning or as tired as we hope to be when it is time to sleep.

Sleep deprivation affects the circadian rhythm which may affect hair growth cycles. One study showed that sleep deprivation resulted in decreased beard-hair growth.[12] The study attributed these effects to lower protein synthesis during sleep deprivation as well as less growth hormone and dihydrotestosterone (DHT) release. On the contrary, there is research in twin studies that found associations with sleeping more to correlate with more hair loss in both the front and side parts of their head.[13] However, the researchers cautioned to not simply conclude that prolonged sleep is what causes baldness. Instead, they were concerned that antidepressant and anti-anxiety medications could also lead to prolonged sleep hours, indicating more of a connection between excessive stress, depression, and hair loss.

There are still questions as to whether or not more sleep allows hair to grow lusher and longer. Regardless, there is good evidence that the hair follicle has a biological clock and it may be connected to the body’s circadian rhythm.


Future Therapies

  • PER1 and BMAL1 are potential targets for therapy in modulating hair growth by avoiding undesired effects on the central clock. By blocking the activity of such genes, future therapies may prevent hair loss (alopecia). By promoting the activity of these target genes, therapy may suppress unwanted hair growth (hirsutism).[4]
  • Melatonin is commonly used to try to regulate the sleep-wake cycles and it may influence the hair growth cycle as well.[14] Topical application of melatonin may be a treatment option for those with female and male pattern hair loss.[15]
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