Sun exposure: what are the effects on hair?

Exposing hair to sunlight can sometimes slightly alter hair colour. In summer in particular, it’s not uncommon to notice lighter highlights, or even a subtle natural lightening that gives hair a more luminous appearance. This phenomenon, often sought for its aesthetic effect, evokes a certain summer lightness. Indeed, some people appreciate the sun’s effect on blond, chestnut or brown hair to achieve golden or copper tones without dye. Yet this change is not trivial: it reflects chemical and structural alterations of the hair fibres. Hair consists of three cylindrical layers: the medulla, the innermost layer; the cortex, which gives fibres their elastic properties and contains the melanin that gives hair its colour, and the cuticle, the external protective layer made of flat keratin cells arranged like tiles.

When hair is exposed to sunlight, the hair fibre tends to absorb UV radiation. Indeed, hair proteins, such as keratin, contain amino acids like tryptophan, tyrosine, phenylalanine, and cysteine, some of which are sensitive to UV rays. Tryptophan, for example, absorbs UVB around 280 nm and is involved in photodegradation. When exposed to this radiation, it transforms into N-formylkynurenine. This yellow compound absorbs at 420 nm. The conversion of tryptophan to N-formylkynurenine is one of the markers used to assess sun-damaged hair. Under UV exposure, disulfide bonds—essential for fibre cohesion and strength and responsible for hair shape (straight, wavy, curly or coily)—can break. This alters hair’s mechanical resistance and makes it more prone to breakage.

In addition to interacting with hair fibre proteins, sunlight leads to reactive oxygen species formation, which represents a secondary pathway of photodegradation. Produced under UVA exposure, these free radicals attack amino acids and contribute to the oxidation of natural melanin or artificial hair dyes. This process can cause colour loss or even yellowing, especially in blond or white hair. Sun-induced yellowing indicates significant pigment degradation. White hair is particularly vulnerable as it lacks melanin, which, beyond its aesthetic role, serves mainly a protective function. It absorbs UV and neutralises free radicals but degrades gradually in the process. That is why hair tends to lighten during summer. It also often becomes dry. Indeed, sun exposure is often associated with moisture loss, especially in curly or coily hair, which is naturally drier.