The contrast of just your screen and focusing in the dark is likely damaging on its own; but you can add the red filter, and, if using kindle app, make the font

The contrast of just your screen and focusing in the dark is likely damaging on its own; but you can add the red filter, and, if using kindle app, make the font white on black background. But yes, as Kindles imply, reading text printed without a backlight is preferable to backlit text, so prefer lights over the backlight.