I would think eye strain. I have two pairs of reading glasses - one for computer work ~ 1.5 diopter and one for book or close up reading on tablet ~ 2.0 diopter.
i find the night mode or other blue filtering makes very little difference to how my eyes feel. I thought the theory was that it suppressed melatonin production and kept one awake.
certainly staring at a computer screen, one can forget to blink and keep the eyes moist.