Conversely, two analyses of the Nurses Health Study (NHS) have demonstrated an association between working night shift (just 3 a month for 15 years or more) and an increase risk in both colon cancer and breast cancer . Another observational study came to a similar conclusion linking night shift work to increased breast cancer risk .
Of course science is never clear cut & crystal clear. Case in point, another analysis of NHS showed that working the night shift was protective against skin cancers, particularly melanomas . Perhaps working night shift prevents ultraviolet radiation's mutagenicity which is more critical for skin cancer while the decrease in melatonin allows colon & breast cancers to develop.
Whatever the explanation, in October 2007, the International Agency for Research on Cancer felt strongly enough about the available data to conclude that " shift-work that involves circadian disruption is probably carcinogenic to humans ".
So what are we to do? Someone has to work those night shifts. And the fact of the matter is that just about everyone of us turns on the lights in our homes after it gets dark inside (and outside). I doubt that we're going to return to our ancestors' ways of working from dawn to dusk and resting (sleeping) from dusk to dawn. But we could at least turn off the lights (and TV) before we go to sleep.
PS Those low wattage/lux night lights don't count. We don't want you tripping and breaking something!