I think the seasonality data may be mixed, with more diagnoses in the fall and spring in human subjects studies.
This particular claim was a hypothesis from Richard Stevens, with some experimental work that shows rapid cancer growth under constant light (in human tumors implanted in mice) by Blask (2002, 2005): http://www.nel.edu/userfiles/articlesnew/NEL230802A03.pdf
The main mechanism in the Blask work is thought to be suppression of melatonin.
The rat study you cite here is certainly far from sufficient evidence to make such bold claims or implicated that indoor lighting can promote cancer development in humans! I think you should edit your statement in the top comment further to make it clear that you are not referring to any solid human data here at all.
Can you point me to a reference for this? This pattern is surprising and I would like to learn more about this.