The results of several studies suggest that women who work at night — factory workers, doctors, nurses, and police officers, for example — have a higher risk of breast cancer compared to women who work during the day. Other research suggests that women who live in areas with high levels of external light at night (street lights, for example) have a higher risk of breast cancer.
Researchers think that this increase in risk is linked to melatonin levels. Melatonin is a hormone that plays a role in regulating the body’s sleep cycle. Melatonin production peaks at night and is lower during the day when your eyes register light exposure. When women work at night or if they’re exposed to external light at night, their melatonin levels tend to stay low.