I recommend you see an ophthalmologist to have a full exam to look especially for anything that could be causing minor eye irritation. You have looked into the usual causes like stress, caffeine, lack of sleep. Perhaps there is something in your diet or routine that has more stimulant than you think like chocolate, tea, cigarettes, supplements, energy drinks, certain pain medications. Good Luck.
I forgot to add that I have horrible vision. I don't know if this has anything to do with it, but my prescription is around -10 on each eye.
Thanks for the help :)