I hate to nitpick, but Kindles still don't really have "backlights"; they're called "frontlights". Basically it's a "light-conducting" layer on top of the screen that's illuminated by LEDs at the top, bottom, or both. It's hard to describe, but the effect is very different from backlit tablets, and much more gentle. It's like this for all e-Readers AFAIK.

It would be interesting to see a study especially since the tech is so different.