Available evidence suggests that blind women have a lower risk of breast cancer than sighted women. The risk is thought to be related to factors influenced by the eye's perception of light, such as melatonin or circadian synchronization. Circadian synchronization depends on an internal clock that is synchronized to light-dark cycles. This study examined the question of whether total visual blindness provides more protection against breast cancer than partial blindness. Women with no perception of light were found to have a significantly lower incidence of breast cancer than women with some light perception. The results were not modified significantly when stratified by menopausal status or shift work, or when excluding participants who were diagnosed with breast cancer within two or four years of the onset of blindness.

Other studies support melatonin hypothesis

Exposing the eyes to light (especially blue light wavelengths) when melatonin synthesis normally occurs reduces or eliminates melatonin synthesis. Evidence has been accumulating since the 1990s that a lack of melatonin might be linked to increased risk of breast cancer. Melatonin has been shown to interfere with estrogen-signaling pathways and reduce aromatase activity and expression in human breast cancer cells, thereby inhibiting breast cancer growth. Having more melatonin in first morning urine has been reported to be associated with a reduction in the incidence of breast cancer.

Almonds, cherries (especially sour or tart cherries), and tomatoes are very good food sources of melatonin. Walnuts, oats, and sweet corn also contain some melatonin. On the other hand, red meat consumption has been shown reduce circulating melatonin. The safety and effectiveness of melatonin supplements have not been established and we do not recommend them for breast cancer prevention.