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.