This is not a controlled study, it must be the famous "Nurses Study." They simply collect all manner of data from Nurses throughout their lives and then find all sorts of correlations. The problem is, it isn't controlled. The Night Shift could be producing cancer because of stress, or artificial light, or air conditioning after dark, or exposure to toxins while sleepy, or unreported sex with doctors (hey, they need to put some explanation on the form, how about "night shift?). The good thing about the study is it allows researchers to see what correlations are there and then come up with hypotheses to test in real, controlled experiments. The downside is that the media gets a hold of the correlations and every week we get a story like "Now Doctors at Harvard are saying that working at night increases your risk of cancer."

There's an example of this in WYSIWYG's original post:

"The researchers studied 78,586 women who worked at least three night shifts per week for 15 yrs. These women had a 35% greater risk of colon or rectal cancer."

In fact, they showed a greater incidence, not a greater risk. Nobody knows if working at night increases the risk. They just know that something about women who work at night makes them get cancer more often than those who don't. It isn't necessarily working at night that does it, it may be a related feature of some kind.