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The Science of Hair Loss
How Hair Actually Grows (And Stops Growing)
The human hair follicle is a complex regenerative organ that develops before birth. Under normal conditions, these follicles repeatedly rebuild their own structures to produce new hair strands throughout life. Hair grows through a repeating biological cycle of growth, transition, rest, and shedding. Understanding this cycle helps explain why some hair shedding is normal, and why certain conditions can thin hair.
At the Center for Hair Health, we explore the science behind hair growth to help you understand what is happening to your hair and when you should seek professional advice.
The Three Phases of Hair Growth
The major classical phases of hair growth are Anagen, catagen, and telogen. Exogen describes the actual shedding of the old hair. The term kenogen is also used to describe an interval when a follicle may remain temporarily empty before a new hair strand emerges.
Unlike animals that often experience a synchronized seasonal molt, human follicles cycle through the various phases independently. So, at any given time, most healthy follicles are actively growing while a few others regress, rest, or shed. This asynchronous behavior ensures you don’t lose all your hair at once. Instead, you shed a few individual hairs daily while maintaining your overall hair density.
Changes in the number of follicles in different phases of the cycle drive many forms of hair loss.
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Anagen: The Growth Phase
This is the active phase of the hair growth cycle. Hair follicles can remain in the anagen phase for years, though the duration varies by person and follicle. At any given time, about 90 percent of your scalp hairs are in the anagen phase. Your hair's maximum length depends on how long the follicle stays in this phase. People whose scalp follicles stay in anagen for years grow longer hair than people with shorter growth phases.
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Catagen
This brief regression phase follows active growth. This phase lasts between one and two weeks. The follicle becomes shorter as it prepares to enter the resting state.
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Telogen
During this phase, the follicle remains relatively inactive. Telogen lasts several months, and the follicle retains the hair until it is ultimately released and a new growth cycle begins. Hair shedding is a normal part of the hair growth cycle and not necessarily a sign of hair loss.
How much hair shedding is normal?
Most people lose 50 to 100 hair fibers daily, though this can vary by hair type, washing routine, and how often you comb your hair.
Normal hair shedding doesn't cause thinning because new strands grow to replace the ones you shed. However, if you notice more shedding, a widening part, a receding hairline, or visible thinning, it could mean something else is happening.
Why does hair start to thin?
Hair thinning has many possible causes, including:
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Hereditary hair loss due to genetics
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Hormonal changes, such as pregnancy, childbirth, or menopause.
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Medical conditions like androgenetic alopecia
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Nutritional deficiencies, especially low protein, B12, or iron.
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Fungal infections on the scalp
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Haircare practices that cause damage due to processing, including bleach and perms
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Hairstyles that pull the hair tightly, including hair extensions and tight ponytails
These different causes explain why no single biological factor causes hair loss. Some disorders affect hormonal signaling, while others force many follicles to enter a resting state prematurely. Another factor may interrupt rapidly dividing cells during the anagen phase. Visible hair loss may look similar, while the biological cause can be completely different.
One common cause of hair loss is androgenetic alopecia, which affects men and women. This condition causes follicles to become smaller and produce short, fine hairs. Over time, the affected areas may look less dense, and the scalp may become more visible.
Some follicles are also more sensitive to hormonal signaling than others. This explains why pattern hair loss affects certain areas of the scalp and not every hair follicle equally.
What is DHT, and How Does it Affect Hair?
Dihydrotestosterone, or DHT, is a hormone produced from testosterone in certain cells through an enzyme called 5-alpha-reductase.
In people with androgenetic alopecia, DHT binds to receptors and can shrink hair follicles, reducing hair density. The affected follicles start producing slightly thinner hair. This process can lead to gradual thinning.
However, DHT doesn't cause all hair loss. Identifying the cause is important because it determines the type of treatment you get.
When Shedding is a Different Story
Telogen effluvium occurs when an unusually large number of follicles enter the resting phase and later shed hair. Major physical or emotional stressors, such as surgery, a significant illness, or rapid weight loss, can cause it.
You may not notice the shedding until a few months after the stressor. In most cases, shedding is temporary, and hair density improves once the underlying cause resolves. However, persistent shedding may need further assessment by a professional.
Telogen effluvium is different from androgenetic alopecia, which typically involves gradual thinning and follicle miniaturization. However, these two conditions can occur in the same person.
If hair loss is sudden, persistent, or accompanied by itching, inflammation, bald patches, or scalp pain, consult a dermatologist or healthcare professional. They can help identify the cause of your hair loss and suggest the best treatment options.
What to Explore Next
Hair health involves more than the hair growth cycle. Explore our other resources to learn more about:
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Treatments and devices: Understand the available hair loss treatments and the evidence behind them.
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Nutrition: Learn how diet and nutritional deficiencies may affect hair growth
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Stress and lifestyle: Explore how stress can trigger shedding
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Medical causes: Discover how health conditions, medications, and hormones can influence hair growth.
Related articles
The Stress-Hair Connection is Real. Here’s the Mechanism
While we often view stress as a psychological issue, periods of significant stress can affect your body and result in a long list of symptoms, including hair loss. During stressful periods, the body releases the hormone cortisol to help us cope. Short-term stress is a normal part of life. However, if stress is long-term or chronic, you may have excess stress hormones circulating in your body. This can lead to negative effects, such as hair shedding. However, stress does not explain every case of hair loss. Some people lose hair due to hormones, genetics, medical conditions, and nutritional factors. Understanding the connection between stress and hair loss can help you respond to the shedding without adding unnecessary pressure to an already stressful situation. Can Stress Trigger Hair Shedding? Yes, significant physical and emotional stress can trigger two types of hair loss, namely: Alopecia areata Telogen effluvium Alopecia areata is caused by severe stress that triggers the immune system to attack hair follicles and cause hair loss. Telogen effluvium is one of the most common results of stress-related hair loss. Potential triggers of telogen effluvium include surgery, serious illness, childbirth, major emotional distress, and rapid weight loss. The effect is not always immediate. Most people notice increased shedding up to three months after a triggering event, making it difficult to connect the hair loss to its original cause. Stress also affects hair health indirectly. Prolonged distress can disrupt sleep and appetite, making it harder to maintain the lifestyle that supports normal hair growth. Understanding Telogen Effluvium Hair follicles grow through a repeated cycle of growth, transition, rest, and shedding. Extreme stress can prematurely shift hair follicles from the anagen phase to the telogen phase. The affected hair then sheds a few months after the trigger. You may notice more hair in the shower, on your hairbrush, or on your pillow. In many cases, telogen effluvium improves once the underlying trigger has resolved. Hair growth recovers gradually, although the timeline varies. Persistent shedding may indicate an ongoing trigger or another condition that needs assessment. Telogen effluvium is different from androgenetic alopecia, also known as pattern hair loss. Androgenetic alopecia involves gradual follicle miniaturization in genetically susceptible people. However, both conditions can occur together, so increased shedding does not rule out another cause of thinning. The Cycle of Hair Loss and Emotional Stress Noticing hair loss can create worry and stress, as you wonder whether the shedding will stop or what your hair will look like in the coming months. These concerns, while legitimate, create a cycle in which your worry about hair loss adds to existing stress. Constantly checking your hair, comparing photographs, and trying multiple products at once can make it harder to feel in control. This doesn't mean your concern is causing the hair loss. It also doesn't mean simply relaxing will reverse hair shedding. It means caring for your emotional well-being is key while investigating the cause of hair loss. Evidence-Informed Ways to Manage Stress Stress management does not guarantee hair regrowth, but practical habits can support your overall health and make difficult periods more manageable. Prioritize sleep: Keep a reasonable and consistent sleep schedule and create a relaxing bedtime routine. Poor sleep can make stress harder to manage. Make time for relaxation: Try slow breathing, meditation, or gentle stretching to help you unwind. Choose a relaxation method that’s realistic and easy to maintain. Balance exercise with recovery: Regular physical activity can support mood and general health. Avoid combining intense training with inadequate food intake or insufficient rest. Address the underlying stressor: When possible, identify what is contributing to your stress and consider practical steps to reduce it. Talking with someone you trust, seeking workplace support, or working with a qualified mental health professional may help. Avoid turning recovery into another source of pressure: You don’t need a perfect routine to support your hair health. Focus on manageable changes rather than trying to control every aspect of your hair growth. When to Seek Professional Help Talk to a licensed healthcare professional if you experience persistent or heavy shedding, gradual thinning, distinct bald patches, or scalp pain, itching, and inflammation. A healthcare professional can assess possible causes and decide whether testing or an examination is appropriate. Seek support from a mental health professional if stress is affecting your sleep or ability to manage daily life. Do not wait until things feel unmanageable to ask for help. Identifying the cause of hair loss matters because treatments differ. Stress-related hair loss often doesn’t require medical intervention, as it improves on its own once the underlying trigger resolves. Other conditions may require specific medical treatment. Stress Management and Hair Loss Treatment Managing stress is important and can support your overall health. However, it is not a replacement for assessing and treating an underlying hair condition. Reducing stress does not guarantee hair growth if stress didn't trigger the hair loss. Visit our Science of Hair page to understand the growth cycle, and explore our Medical Conditions and Medications page to learn about other possible causes of hair loss. Our Devices and Treatments page can help you review available treatment options and the evidence behind them.
Nutrition For Stronger Hair
What You Eat Shows Up In Your Hair, Eventually The right nutrition provides the building blocks, scalp protection, and oxygen delivery needed for strong hair structure, active follicles, and hair growth. Your follicles are active structures that need a steady nutrient supply. Nutritional deficiency can contribute to shedding and changes in hair quality. However, eating well alone does not guarantee you will not experience hair loss. Other factors like genetics, hormones, medications, and medical conditions can influence hair growth and density. Understanding nutrition's role in your hair journey can help you make informed choices without spending money on supplements you may not need. Why Hair Needs Adequate Nutrition Hair production requires protein, energy, vitamins, and minerals. When your body doesn’t get enough of certain nutrients, hair growth may suffer. The right nutrients can also support active hair growth by fueling the high metabolic rate and rapid cell division anagen follicles need. For example, severe calorie restriction and inadequate protein intake can increase hair shedding. The goal of nutrition and a balanced diet is to meet your body’s needs through varied foods, rather than focusing on a single so-called superfood or relying on products marketed for hair growth. Protein and Keratin Hair is primarily made of keratin, which is a structural protein. Your body needs this dietary protein to build and maintain this structure. Good protein sources include eggs, fish, meat, milk, peas, nuts, and lentils. Include a variety of protein-rich foods to meet your nutritional needs. Protein deficiency can cause hair shedding. However, eating more protein than your body needs has not been shown to accelerate hair growth or reverse hair loss. Iron and hair shedding Iron helps your body produce hemoglobin, which carries oxygen in the blood. Low iron levels, including iron deficiency with or without anemia, have been associated with certain types of hair loss, particularly diffuse shedding. Good sources of iron include lentils, beans, fish, meat, and iron-fortified foods. Vitamin C-rich foods can help your body absorb iron from plant sources. If you suspect iron deficiency, a healthcare professional can assess your symptoms and determine whether blood tests are appropriate. Do not take iron supplements without your clinician's guidance, as excess iron can be harmful. Vitamin D and Hair Health Vitamin D supports several bodily processes, including normal hair follicle function. Research has found associations between low vitamin D levels and certain types of hair loss, but these associations do not prove that low vitamin D causes every case. Foods rich in Vitamin D include oily fish, egg yolks, and vitamin D-fortified products. If you have a deficiency, a healthcare professional will advise whether taking a supplement is appropriate. Zinc Zinc plays a vital role in cell growth and tissue repair. Zinc deficiency can contribute to hair loss, but that doesn't mean you should take a zinc supplement if you experience hair thinning. Talk to your healthcare provider first, as excessive zinc supplementation can cause side effects and interfere with copper absorption. Foods rich in zinc include dairy products, shellfish, meat, beans, seeds, and nuts. Biotin Biotin, also known as Vitamin 7, is a crucial vitamin that helps the body process nutrients. It also plays a role in keratin production. You can get biotin in your normal diet by eating eggs, seeds, fish, meat, and nuts. Biotin deficiency can cause hair thinning, but it is uncommon. Extra supplementation can only improve hair growth if you have an actual deficiency. High-dose biotin can also interfere with certain laboratory tests, including some tests used to assess hormone levels. Tell your healthcare provider about any biotin supplements you take, especially before having blood tests. Do hair growth supplements work? If you have a confirmed nutritional deficiency, supplements may make a meaningful difference. To create a health plan, contact a healthcare provider to check your levels with a blood test. Unfortunately, most supplements that are marketed specifically for hair growth have limited or inconsistent supporting evidence. Hair growth supplement formulas are generally designed to work for a broad range of people, so they don’t necessarily address the root cause of your individual hair loss. Blanket supplementation for people without a deficiency is discouraged. This is because excess amounts of certain nutrients, including selenium and vitamin A, can actually cause hair loss. Some supplements may also interact with medications and cause other health problems. Before buying any hair growth supplement, check its ingredients and discuss its suitability with a qualified healthcare professional. Be wary of any products that promise rapid or guaranteed results. When to seek testing or medical advice Consider talking to a medical professional if you experience persistent shedding, noticeable thinning, or sudden changes in hair density. The healthcare professional can review your diet, medical history, medications, and symptoms to help identify possible causes. They may order blood tests depending on your circumstances. These could include tests for iron status or other nutrients if they suspect a deficiency. Testing is guided by your symptoms and risk factors, not by assuming every case of hair loss is caused by a nutritional deficiency. Nutrition is a foundation, not a fix. A balanced diet supports normal hair growth, but nutrition is only one part of hair health. Correcting a deficiency may address shedding related to that deficiency, but it will not necessarily reverse pattern hair loss or treat other underlying conditions.