Background. By suppressing secretion of melatonin, outdoor artificial light at night (ALAN) may increase risk of breast cancer. Previous studies of ALAN and breast cancer have produced mixed results, which may be attributable to ALAN measures that did not account for wavelength of light; suppression of melatonin by ALAN is influenced by both light intensity and wavelength. Methods. We conducted a study of 318 breast cancer cases and 3180 controls nested within the British Columbia Generations Project (BCGP). Measures of ALAN were generated from colour images captured from the International Space Station (ISS) and linked to participant residential addresses. Measures included visual radiance (i.e., brightness), melatonin suppression index (MSI), which quantifies the extent to which light spectra suppress melatonin, and the product of visual radiance and MSI (Impact MSI). Unconditional logistic regression models, adjusted for matching factors, were used to evaluate associations between ALAN exposure and breast cancer risk. Odds ratios (OR) with 95% confidence intervals (CI) were generated as estimates of relative risk. In exploratory analyses, we evaluated associations with individual breast cancer hormone receptor subtypes as well as associations when excluding participants that reported working night or rotating shifts at time of recruitment and when restricting to participants reporting light entering their rooms when sleeping. Results. No statistically significant associations between ALAN measures and breast cancer risk overall or with specific tumour receptor subtypes were observed. In analyses restricting to those participants reporting light entering their rooms while sleeping, a statistically significant 42% increase in risk of breast cancer was seen when comparing the highest to lowest tertiles of Impact MSI (95% CI: 1.05-1.92). Conclusions. While we did not find evidence of an overall relationship between outdoor ALAN and breast cancer risk, exploratory analyses restricted to participants reporting light entering their rooms while sleeping suggested an increased risk of breast cancer among those most highly exposed. This association was specifically found with an ALAN measure that accounted for both aspects of light exposure that impact melatonin secretion: intensity and wavelength. To confirm these findings, a larger-scale pooled study using similarly collected data from other cohorts is warranted.