I don’t think anyone really knows that for sure, snowdrop. I know that I read lunaception (which is a book), which was highly unscientific, but she surmised it would be different than darkening your room completely. There is also work a nurse has done on sleeping in darkness and cycle problems, but I have heard the book isn’t very great.
Book: http://onemoresoul.com/catalog/effects-of-light-on-the-menstrual-cycle-and-fertility-p316.html
Page that has a link to pdf: http://onemoresoul.com/news-commentary/medical-matters-the-effects-of-light-on-the-menstrual-cycle.html