I used to do something similar to you - 12 hours coding every day, with a couple of hours of Counterstrike, for about 3 years. I thought I was being very productive. But then I started to measure my effectiveness (purely out of interest rather than any belief I wasn't working well) - I measured my billable hours, lines of code, commits, issues fixed, etc. Over the course of about 3 months I found that I did less work when I was in front of the screen for 12 hours compared to 8 hours. Spending more time in front of my computer reduced my effectiveness. I put the reasons down to the fact I could do exercise, sleep better, have a social life, etc - by being healthier I work better. That said, my Counterstrike skills completely disappeared, so there were downsides.
My suggestion would be to take a few hours to build something that tracks how well you work, and then vary the hours you put in for a while. See what happens.
Regarding your eyes, try to make your screen brightness match the ambient level of light in your environment, sit at a distance where you don't need to actively focus (use glasses if necessary), and pick a theme for your IDE that's quite dark. The less work your eyes have to do the better.