The Nocturnal Brain: Nightmares, Neuroscience, and the Secret World of Sleep

by Guy Leschziner

So why should shift-workers have increased rates of certain cancers? One hypothesis is centred around the exposure to light at night. As we’ve discussed, light exposure at night suppresses the production of melatonin by the pineal gland, and it is argued that melatonin may have some anti-cancer activity above and beyond its role as a hormone — specifically, to absorb toxic by-products of oxygen metabolism that are thought to damage our DNA and predispose us to cancer. So, by regular exposure to light at night, perhaps we are lessening our resistance to cancer. This hypothesis is supported by the fact that people who are totally blind are less likely to develop breast cancer than normally sighted individuals, and in one experiment mutant mice predisposed to breast cancer were more likely to develop tumours when their circadian rhythms were disrupted. But there are lots of potential confounders.