How sunless tanning works

The reaction

Sunless tanning relies on dihydroxyacetone, a simple sugar. Applied to skin, it reacts with amino acids in the outermost layer — the stratum corneum — in what chemists call a Maillard reaction, the same browning chemistry that colours bread crust. The result is a group of pigments called melanoidins.

Nothing penetrates deeper than that surface layer, and no melanin is produced. The colour is a surface stain that sheds with the skin cells holding it.

Timings

Colour begins to appear within two to four hours and continues developing for up to twenty-four. Peak depth is usually somewhere between eight and twelve hours. It then fades over roughly five to ten days as surface cells shed.

Why tans go patchy

Almost always preparation. Dry patches — elbows, knees, ankles, knuckles — hold more product and go darker. Residual moisturiser, deodorant or oil blocks colour and leaves pale gaps. Freshly shaved skin can take up colour in the follicles as dots. Uneven shedding afterwards turns even colour into a patchwork.

Getting it even

Exfoliate the day before, not the same day. Arrive with completely clean, dry skin. Shave the night before. Use a light barrier cream on the driest areas. Afterwards, moisturise daily so shedding is slow and uniform, and avoid long hot baths and chlorinated water while the colour holds.

One important caveat

A sunless tan offers essentially no protection against ultraviolet radiation. It is cosmetic colour, not a barrier. Sunscreen is still required, and the darker appearance can make it easy to forget that.