1. Levels of artificial night lighting are increasing rapidly worldwide, subjecting nocturnal organisms to a major change in their environment. Many moth species are strongly attracted to sources of artificial night lighting, with potentially severe, yet poorly studied, consequences for development, reproduction and inter/intra-specific interactions.2. Here, we present results of a field-based experiment where we tested effects of various types of artificial lighting on mating in the winter moth (Operophtera brumata, Lepidoptera: Geometridae). We illuminated trunks of oak trees with green, white, red or no artificial LED light at night, and caught female O. brumata on these trunks using funnel traps. The females were dissected to check for the presence of a spermatophore, a sperm package that is delivered by males to females during mating.3. We found a strong reduction in the number of females on the illuminated trunks, indicating artificial light inhibition of activity. Furthermore, artificial light inhibited mating: 53% of females caught on non-illuminated trunks had mated, whereas only 13%, 16% and 28% of the females that were caught on green, white and red light illuminated trunks had mated respectively. 4. A second experiment showed that artificial night lighting reduced the number of males that were attracted to a synthetic O. brumata pheromone lure. This effect was strongest under red light and mildest under green light.5. This study provides, for the first time, field-based evidence that artificial night lighting disrupts reproductive behaviour of moths, and that reducing short wavelength radiation only partly mitigates these negative effects.