I'm not a doctor or such, but the argument I heard for that is that books don't have backlighting, so the surface brightness follows the general brightness in t

I'm not a doctor or such, but the argument I heard for that is that books don't have backlighting, so the surface brightness follows the general brightness in the vicinity, therefore looking at it continously does not strain the eyes as much as looking at a back-lit surface like a screen. (This is also frequently brought up as a benefit of e-ink displays over LCD/LED displays for reading.)