Eyestrain using my iMac

I have an iMac which has retina display. Recently I've noticed that I'm suffering eyestrain (especially in the evenings). I realise that I can turn the Display down in Preferences but I was wondering if there were other settings I could tweak to make things better. I'm not sitting at my iMac right now but I seem to remember there were colour alternatives to Mac colour (an RGB setting maybe?) and other possible variations, many of which I don't recognise/don't know about. I've used PCs with all kinds of (very bright) screens for many years so I know all about taking regular breaks, focusing elsewhere every 20 minutes or so, not sitting too close to the screen and so on. Anything anyone can suggest to reduce eyestrain by altering the Display/other settings would be very much appreciated. Thank you.