(Haha. This is the where I started to think about this question. Because, I think, concrete reasons for cancer are still unknown, so people, out of fear, draw conclusions very quickly. Every other thing is responsible for cancer. One research comes with a positive answer and the other research slams that down. But anyways, my thinking does not change the reality, so let us see what the latest research has to say? Because the cancer is no joke, and we should know the best and latest knowledge in this case)
According to an eye care research conducted under the supervision of Dr. Celia Sánchez-Ramos, there are bad effects of a large amount of blue light for long periods of time. Here are the other researches on vision, done by the same doctor.
Another research done by Richard G. Stevens who has been studying effects of artificial lighting on human physiology for more than three decades has concluded that exposure to blue light before sleeping suppresses melatonin production, a necessary hormone for the human body. Its reduction reduces the quality of sleep. This increases the risk of high blood pressure, diabetes, decreased immunity, and even in some cases, cancer, especially breast cancer (although, the link is not proven).
Zoltan Koltai, EMEA Technology Director, GE Lighting clears some myths about the relation between blue light and cancer in this latest GE blog entry. He, basically, emphasis that same point that we are saying. Blue lights in the evening, or before sleep is not good. But in any way, it is NOT been proved that it causes cancer. LED being a new technology has become an easy target for criticism due to the hype and at the same time the lack of understanding.
The same point is being emphasized in this Harvard study: Keep yourself away from the blue light in the evening. If you subject yourself to blue light for 6.5 hours, it will keep you awake for 3 more hours, even after you have turned off the lights completely.