Monitor brightness down, ambient light up. Monitors are promoted as having crazy brightness, but this is exactly what you don't want for long hours of staring at the screen. You need it as dim as possible without making it hard to see. I run mine at 20% of the factory default.

And of course turn the lights on and/or open a window. No more cave coding.

My other suggestion is sunglasses for every moment you spend in sunlight. This makes a huge difference.

The bottom line is to simply quit making your eyes adjust to extreme swings in brightness.