Greater exposure to light at night raised Body Mass Index (BMI) and waist size in more than 113,000 women taking part in the British study, scientists found.
The Breakthrough Generations Study followed the women for 40 years in an attempt to identify root causes of breast cancer. Obesity is a known risk factor for the disease.
Professor Anthony Swerdlow, from The Institute of Cancer Research in London, said: "Metabolism is affected by cyclical rhythms within the body that relate to sleeping, waking and light exposure.
"The associations we saw in our study between light exposure at night and obesity are very intriguing. We cannot yet tell at this stage what the reason for the associations is, but the results open up an interesting direction for research."
Co-author Dr Emily McFadden, a visiting researcher at the institute, said further investigation into the link between light and metabolism was needed."
The study was funded by Breakthrough Breast Cancer. The charity's senior research officer Dr Matthew Lam said: "It's too early to suggest that sleeping in the dark will help prevent obesity, a known risk factor for breast cancer, but the association is certainly interesting."