Checklist for people with tired eyes:
1) Brightness
Your monitor's brightness might be too high, tone it down a little. Another related solution that could alleviate the situation: change the documents to inverted colors. White text on a dark background is usually easier on the eyes. (Though not sure how common feature that is on the Windows side. On Linux it's quite common.)
2) Font size (/resolution)
Your eyes might be over-worked because you have to focus on a small text. With LCDs it's not recommended to change resolution, as it will make the image blurry. Therefore, it's more recommended to change the default font size (or global DPI), or alternatively use the zoom on webpages and documents. Granted, if the image is too blocky, it might also draw your attention away from working. While that won't be harder on the eyes, it could still slow down the workflow and reduce overall convenience. If buying a new monitor, you should pay attention to a better resolution vs. size combination.
3) Reflections / light-sources
Especially glossy panels can result in reflections, which will make your eyes work harder because they have a harder time focusing. Unfortunately, the #1 is in direct controversy with this, as the reflections will be more noticeable on a dark background. Badly directed lighting usually makes the reflections more noticeable. Try to eliminate lights which are causing the reflections, or get a matte screen. But do note: some people's eyes actually have trouble focusing because of the too aggressive anti-glare matte surface. This was a problem at least two years ago with some Dell monitors, though nowadays they're mostly semi-matte (or light matte, or whatever), which hasn't gotten any bad flack, AFAIK. If possible, you could loan a matte surface monitor from a friend to check if this is indeed a problem for your eyes.
4) PWM
Pulse width modulation will cause tired eyes and head-aches to some people. You can alleviate this by increasing the brightness on your monitor, which makes the duty-cycle of the PWM to increase, thus making it easier on the eyes (/nerve system). Unfortunately, this again makes the #1 more profound, therefore a double-edged solution. Though the inverted colors -solution will still work. PWM-free monitor will naturally be the ultimate choice.
5) 1 minute breaks every 30 minutes
Let your eyes relax by staring at a distance (50-100m) every now and then. Looking out of the window and letting your mind go to a daze will work wonders. Unfortunately again, keeping an open window next to your workstation will often cause the unwanted reflections mentioned in #3.
6) Color temperature
I normally wouldn't recommend this, as it changes the overall color accuracy and thus it's too much of a hassle. But in any case, some people report that "Warm temperature" (which gives a more reddish hue to the image) is more comfortable to the eyes, when compared to "Cold temperature" (which gives a more bluish hue). Though in my opinion, both are usually over-exaggerated in the monitor options, and the best option is somewhere between them. Some monitors even offer a "Normal temperature". Though the earlier mentioned f.lux software apparently takes the hassle out of the equation. It's also apparently a freeware, so no harm in trying.
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And indeed, the BenQ GW2760HS was for a long time even on my wishlist, because it has made all the right choices: lower resolution (1920x1080), adequate size (27"), non-PWM, light matte screen, and it's even very affordable (£216). Unfortunately, the emphasis is on the part "a long time". The BenQ fumbled too long with the release, so I moved on and already got myself another monitor (HDTV). Also, the 2760HS is a VA panel, so there's a potential concern for the gamma-shift.
But now that I looked more into it, nowadays there seems to be multiple models of the 2760. Initially, the HM didn't offer non-PWM, while the HS did. Now there's a GW2760 (without the HM or HS), which seems to be a cheaper "no-frills" option to the other two, but still offers non-PWM. It's even on OCUK's "this week only" deal for £164.
Well, it seems the HS has these features that the budget-GW2760 doesn't at least advertise:
a) a slimmer bezel
b) Color Shift-free Technology (meant to decrease gamma-shift)
c) HDMI port
Personally, I'd advice to go for the HS model, even though it's slightly more expensive (£216 vs. £164).