Epidemiological studies have shown totally blind women have about half the incidence of breast cancer as blind women who retain light control of melatonin suppression or of women with normal vision. Studies show shift worker that reduces melatonin and disrupts the circadian rhythm increases the risk of breast cancer. Studies show human breast cancer grafts grown on rats but supplied with human blood grow rapidly if the blood does not contain melatonin but grow slowly or not at all if the blood contains melatonin. Studies with small rodents show long nights that maximize melatonin can reduce or prevent the growth of breast cancer. Extending the benefit of maximizing melatonin by light control should be tested in humans.