A decades-old belief that sleeping with too much light can increase breast cancer risk is a myth, research suggests.
Since the 1970s, exposure to light at night has been proposed as a risk factor for the disease, due to disruption of the body's internal biological clock.
But scientists who studied 105,866 British women for six years found no association between night-time light levels and breast cancer risk.
The 1775 women who went on to develop the disease were no more likely than the others to have been exposed to high light levels while asleep.
Professor Anthony Swerdlow, from The Institute of Cancer Research, London, said: "The theory that exposure to light at night might affect a woman's risk of breast cancer dates back 30 years. It remains an intriguing but unproven hypothesis.
"Our study suggests that light levels at night do not materially increase a woman's risk of breast cancer, but further research will be needed before a definitive conclusion can be reached."
Circadian rhythms - the system of "clocks" that affect a multitude of biological processes - are influenced by external factors such as light and temperature.
During a normal sleep-wake cycle, reduced light levels trigger secretion of melatonin, the "sleep hormone".
Previous studies have indicated that by lowering melatonin, exposure to night-time light may boost circulating oestrogen levels and increase breast cancer risk.
All the women were participants in the Breast Cancer Now Generations Study, which has been investigating the causes of breast cancer for 40 years.
Adjusting for different sleep patterns, sleep duration and night shift work did not appear to affect the results. The findings appear in the British Journal Of Cancer.