Did someone mention f.lux yet?
I didn't mention Flux, and on purpose. Makes it so much worse for me and pretty much everyone with eye conditions I've talked to. Said to help a few people whose eyes are good but if you're experiencing focusing issues, I don't think a screen that gradually turns another color is going to do much to help the issue, not at all.
I see
F.lux doesn't gradually turn another color. At a certain time it is an instant switch (1-2) seconds to the different color.
For me it is a big help for eye strain at night to not be looking at the white screen
It's probably the LED backlight. They have a much lower refresh rate than CCFL backlights (500Hz vs 5000Hz or so). Newer ones tend to be better, but still hurt my eyes after a while.