Hair and Nails Explained: Growth, Cycles and Colour
Why hair grows and sheds in cycles, why nails keep growing, and what they are all made of.
Hair and Nails: More Than Meets the Eye
Hair and nails might seem like minor accessories — useful for warmth or practical tasks, perhaps, but not particularly interesting biologically. In fact, both are complex, living-derived structures with their own growth cycles, cellular mechanisms, and evolutionary purposes. They share a common protein scaffold, grow from specialised structures embedded in the skin, and respond to health, hormones, and nutrition in ways that can provide clues about what's happening inside the body.
Both hair and nails are closely related to skin, so this post pairs naturally with our human skin guide and the hair biology guide.
What Hair and Nails Are Made Of
The main structural material in both hair and nails is keratin — a family of fibrous proteins also found in the outermost layer of the skin (the stratum corneum) and in animal horns, claws, and feathers. Keratin molecules form coiled chains that then bundle together in a helical arrangement, giving the final structure high tensile strength and considerable resistance to mechanical stress.
Hair keratin and nail keratin differ slightly in their composition. Nail keratin is harder and more tightly cross-linked — due to a higher number of sulphur-to-sulphur chemical bonds between protein chains. Hair keratin is more flexible, allowing strands to bend without breaking.
Both structures are largely composed of dead, keratin-filled cells by the time they emerge from the skin. The visible part of a hair or nail contains no living cells and no blood vessels. All the active biology occurs at the base, hidden within the skin.
The Structure of a Hair
A single hair consists of two main parts. The hair shaft is the visible portion — the strand above the skin surface. The hair follicle is the living structure embedded within the skin that produces it.
The shaft itself has three layers. The medulla is the soft, innermost core (sometimes absent in fine hairs). The cortex is the thickest layer, made of tightly packed keratin fibres aligned lengthwise — this is where pigment granules are deposited and where the hair's mechanical strength comes from. The outermost layer is the cuticle: a single layer of flat, overlapping scales like roof tiles. Healthy cuticle scales lie flat, reflecting light and giving hair its shine; damage (from heat, chemicals, or rough handling) lifts these scales, making hair look dull and feel rough.
The follicle at the base of each hair houses a small cluster of stem cells called the bulge, which provides the cells needed for continued hair production. At the very bottom of the follicle is the dermal papilla — a small structure rich in blood vessels and nerve endings that supplies nutrients to the growing hair and regulates the growth cycle. A sebaceous (oil) gland is attached to most follicles, lubricating both the hair and the surrounding skin surface.
The Hair Growth Cycle
Each hair follicle operates independently and cycles through three phases. At any given time, hairs across the scalp are at different stages of this cycle — which is why we shed hair continuously without going bald.
| Phase | Name | Duration | What happens |
|---|---|---|---|
| Growing | Anagen | 2–7 years (scalp) | Hair shaft is actively produced; follicle is deep and well-supplied |
| Transition | Catagen | 2–3 weeks | Follicle shrinks; hair shaft disconnects from dermal papilla |
| Resting / shedding | Telogen | 3–4 months | Hair rests then sheds; new anagen hair begins growing beneath |
Scalp hair grows roughly 10–15 centimetres per year (about 1–1.25 cm per month). The length of the anagen phase determines maximum hair length — people whose anagen phase lasts 7 years can grow hair much longer than those whose anagen phase lasts 2 years. Genetics largely determines this.
It is normal to shed about 50–100 hairs per day from the scalp. This can seem alarming in the shower, but it represents only a tiny fraction of the approximately 100,000 hairs on a typical scalp. Sudden increases in shedding — for example after illness, surgery, nutritional deficiency, or major stress — can cause noticeable thinning, but the hair usually regrows once the underlying cause resolves.
What Determines Hair Colour
Hair colour is determined by melanin — the same pigment responsible for skin and eye colour. Melanocytes in the hair follicle inject melanin granules into the cortex cells of growing hair shafts.
There are two types of melanin that combine to produce the full range of natural hair colours. Eumelanin produces brown and black tones; more eumelanin means darker hair. Phaeomelanin produces red and yellow tones. The relative proportions of these two pigments, along with the overall concentration of melanin, produce the spectrum from platinum blonde to jet black, with red and auburn as intermediate mixtures.
With age, melanocytes gradually become less active and eventually stop producing pigment, causing the characteristic greying of hair. The timing is largely genetic — grey hair onset tends to run in families.
The Structure and Growth of Nails
A nail consists of a nail plate — the hard, visible part — that rests on the nail bed, a layer of skin beneath. At the base of the nail plate, mostly hidden under the skin fold, is the nail matrix: the living tissue that generates the cells which become the nail plate. New cells are constantly produced in the matrix and push older cells forward, compressing them into flat, keratin-filled sheets that form the visible nail.
The pale crescent shape visible at the base of some nails — the lunula — is the visible part of the nail matrix. It appears white because the matrix tissue is thick enough to obscure the pink of the underlying blood vessels.
Fingernails grow roughly 3–4 millimetres per month; toenails grow more slowly, at about 1.5 mm per month. Nails typically grow faster in summer (believed to relate to improved circulation in warmer temperatures) and faster on the dominant hand. Fingernails on longer fingers tend to grow faster than those on shorter fingers.
Nails serve practical functions: they protect the sensitive fingertips, enhance fine touch sensation by providing a stable backing for the fingertip's touch receptors, and assist with picking up and manipulating small objects.
What Nails Can Reveal About Health
Clinicians sometimes examine nails as part of a health assessment because their appearance can reflect systemic conditions. Some patterns are well-established; others require professional interpretation in the context of the whole person.
- Pale or white nails can sometimes be associated with anaemia, liver disease, or other systemic conditions — but also simply indicate a naturally less-pink nail bed.
- Clubbing (enlarged, rounded fingertips with nails curved downwards) can be associated with long-standing cardiovascular or respiratory conditions.
- Beau's lines — horizontal ridges across the nail — can mark a period of physical stress or illness during which nail growth temporarily slowed.
- Spoon nails (koilonychia) — concave nails — can be associated with iron deficiency anaemia.
Nail changes alone are rarely diagnostic — they need assessment alongside other clinical findings. If you notice a significant change in your nail appearance, a doctor or dermatologist is the right person to consult. Also see our nail growth guide for more detail.