In the study, researchers took groups of female rats and exposed each group to different intensities of white light during the dark phase of their daily cycle (typical lab rats live in a programmed 12-hour light/12-hour dark cycle). After two weeks of this night cycle disruption, the researchers implanted a tumor (derived from human breast cancer tumors) in the female rats, and continued on with the night cycle disruption for many weeks. By the end of the experiment, the rats that had been exposed to the strongest intensity of light showed a marked increase in tumor growth rates. The brighter the light at night, the bigger the tumor.
Ok, light at night makes tumors grow faster, but can too much light be the cause for cancer? To answer this question, it’s best to turn to studies in humans. There is convincing evidence that women who work night shifts have a significantly higher risk of breast cancer. As well, women with the brightest bedrooms also have a higher risk of breast cancer. Scientists believe the reason for these correlations is a molecule called melatonin. At night, in the dark, your body produces melatonin, which is a very effective anti-cancer molecule. Several studies have looked at this link in more detail and have shown that melatonin can block the development and the growth of tumors in non-human models of breast cancer.
The light-cancer link is gaining interest, and researchers even started sprucing things up by using a catchy acronym, LAN (for light-at-night), so it’s something to keep in mind. Based on this research, I’ve decided to break my habit of flicking on the lights for my midnight nature calls. Would this habit necessarily give me cancer? No. But flicking on the lights does interrupt my production of melatonin, and on top of being an anti-cancer molecule, it’s also a powerful antioxidant. So I’m just trying to put all the chances on my side. That being said, I’m running into a different problem, which is waking up everyone in the building when I stub my big toe on the door frame. Nobody said staying healthy was easy…
Reference: Circadian stage-dependent inhibition of human breast cancer metabolism and growth by the nocturnal melatonin signal: consequences of its disruption by light at night in rats and women. (2010) Blask D.E. et al. Integrative Cancer Therapies, 8(4):347-353