Greater exposure to light at night raised both 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 the institute, said: "Because all the information was collected at the same time, we cannot tell the sequence of events, but the associations we found are consistent with previous research examining light exposure and metabolism, and further investigation is needed.''
The study was funded by the charity Breakthrough Breast Cancer, whose senior research officer Dr Matthew Lam said: "These findings add weight to previous results from animal studies that looked into how light exposure, circadian rhythms and metabolism could all be connected in some way.
"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."
The findings are reported in the American Journal Of Epidemiology.