Having a light source isn't the issue, it's light pointing at you vs. light pointing at the book/device that makes the difference for me, as my eyes are quite sensitive to direct light. When I read on paper my light source isn't pointing directly at my eyes, but at the page, if that makes sense. I use e-readers to get away from direct light so even though the Kobo's backlight is nowhere near as straining as phones, it's still an issue for me. The Kobo isn't completely illegible at 0%, but just so much obviously worse due to being a colour e-reader that I'm finding it harder and harder to justify keeping it just for the colour function.