There's also evidence that our grand transformation of the night might have serious implications for our own health. The contrast between dark and light allows our bodies to calibrate our circadian rhythms, such as hormone levels and sleep schedules. Disrupting these can have a dramatic impact. Researchers think this phenomenon may help explain higher breast cancer rates in societies with brighter nights. Studies on shift workers exposed to nearly constant light during the night hours reveal a higher risk for the disease, perhaps because of altered levels of melatonin. Other research shows that blind women have a lower occurrence of breast cancer. One study that looked at a general population also found a correlation between neighborhood nighttime light levels and breast cancer incidence.