"The advent of electric lighting at night is one of the major reasons why the cancer rate has increased substantially during the last 100 years. The reason for this -melatonin. This hormone, which puts us to sleep, is a powerful anti-oxidant and cancer preventive agent. Results of in vitro and in vivo studies, suggest that melatonin helps the immune system to eliminate neoplastic cells, before cancer develops.
What to do about this? Sleeping in complete darkness is very important, of course. If your bedroom is not dark, wearing a sleeping mask keeps your melatonin levels high throughout the night. Interrupting the darkness, however, for example by turning on a bathroom light, even for a few minutes, can turn off your melatonin for an hour or more! That's why for many people it's hard to go back to sleep after getting up to go to the bathroom."
From: Sentient Cosmos