why are my eyes become so uncomfortable looking at digital screen?

Hello. In the past, my eyes used to feel comfortable unless I watched 3-4 movies straight or something. But starting 10 yrs ago, I noticed that my eyes get uncomfortable looking into digital screen (computer, TV, phone, etc) faster and faster. These days lots of people sit in front of computer at work place for 8 hrs daily and they go home and watch TV or stare at phone, tablet, etc for another 2-3 hrs maybe. And most peoples seem OK with it. 

But, for me, at my work place, my eyes get uncomfortable looking at monitor more than 2-3 hrs. I can't watch TV more than 3 hrs. For some reason, my i phone is worst. My eyes would get uncomfortable after staring at it for 20 mins. So, even at work, I try to minimize monitor viewing AMAP. If I need to read articles, I find myself printing it out and read instead of reading it from monitor. Whenever I don't need to use computer at work, I turn the monitor off. 

I read that some peoples think this is due to not closing my eyes to lubricate while looking at screen. But, I don't really think that's the case for me because i can read paper book all day long without being uncomfortable. I think it's the electrical light coming out of screen that is bothering my eyes. Even at home, I prefer lowering room light and my wife complains why I make room so dark. What is going on with my eyes? Is this just part of aging(i'm 40's)? is that why lots of old peoples wear shade even  indoor because their eyes can't handle too much light? thanks.