Hair gets its natural color from melanin made by pigment-producing cells in hair follicles. As follicles age, those cells and the stem cells that replenish them can become less effective or depleted. New hair then grows with less pigment and looks gray or white. Genes strongly influence when this process begins.
Color is added as a hair grows
Melanocytes in a hair follicle produce melanin and transfer pigment into a growing hair shaft. The strand you can see is not living tissue, so a hair that has already grown out does not gradually change its pigment from the root. The color of new growth reflects how much pigment the follicle supplies during that growth cycle. [1]
Aging changes pigment-producing cells
With age, melanocytes may function less reliably, and the stem-cell population that helps replenish them can decline or lose its normal cycle of renewal. When a follicle supplies little or no melanin, the new strand appears gray or white. The timing differs among people and is influenced substantially by inherited factors. [2]
Stress research needs careful interpretation
Experiments in mice found that intense stress can activate nerves that deplete melanocyte stem cells and cause fur to regrow without its usual pigment. That result identifies a biological pathway in an animal model; it does not show that every gray hair in a person was caused by stress. Human hair graying has multiple influences, and the evidence should not be reduced to a simple stress story. [3]
Why one person grays earlier than another
Follicles do not all lose pigment at the same time, so gray hairs can appear gradually and unevenly. Age, genetics and individual biology shape the pattern. A change in hair color alone is common, but sudden changes accompanied by other symptoms are a separate question for a clinician.
Sources and evidence
- NIH Research Matters — Aging Melanocyte Stem Cells and Gray Hair ↗
NIH summary of research on how aging can affect pigment-producing cells and their stem cells.
- The Biology of Human Hair Greying — PubMed ↗
Review of the pigmentary unit of human hair follicles and biological mechanisms associated with graying.
- Nature — Hyperactivation of Sympathetic Nerves Drives Depletion of Melanocyte Stem Cells ↗
Mouse research on a stress-related pathway; findings are identified here as animal-model evidence.
Sources consulted: 14 October 2026. Written with AI assistance and checked against the linked references; not independently reviewed by a subject specialist. Sources do not endorse this article. See our editorial policy.
Revision: Initial publication; animal research is clearly distinguished from evidence about human hair graying.