I had the same problem until someone gave me the following advice:
If your headaches are behind the eyes, it's probably from squinting as mentioned above. Turn your monitor contrast up as high as it will go, then
reduce the brightness to as low as you can while still being able to read text comfortably. Relax your eyes so that you're not squinting at all. If
you find you're still needing to squint, reduce the brightness further and the contrast if required. Your eyes adjust to the light from a computer
screen just as they do in low light or bright light. You'd be surprised how what seems too dim at first can become quite comfortable to read after a
minute or two of letting your pupils adjust. You should reduce the brightness as much as you can and adjust the contrast to make text more distinct.
Adjust again if you change to viewing sites with bright-colored or white background.
If the headaches are in the back of your head, it may be due to bad seating posture. If you are leaning your neck forward for long periods of time,
you will get a headache from your neck supporting the weight of your head (the human head is actually quite heavy, we just don't use all the stuff in
it to its full capacity
). Slide your chair (or your butt, if the chair is fixed) closer to the computer and lean back so that your back is
straight. Imagine your back is tied to a flat board. Place your feet flat on the floor with your knees at 90 degree angles. If you can, raise the
monitor higher so that you only have to tilt your head down slightly to see it. You can put it on a few large books under it if need be.
Every 10 minutes or so, stop reading, sit up straight, close your eyes and do some neck exercises with slow, relaxed movements between massaging your
neck. Exercises should tilt your head along all axis of movement (first tilt left and right like a quizzical dog, look left and right like you're
crossing the road, and nod back and forth). When massaging your neck, massage upwards to the top of your head rather than downward towards your back.
Stretch your arms and back by trying to "touch the ceiling" while seated. DOing this standing up is even better.
Finish up by massaging your eyes, eyebrows and forehead. Massage your forehead and the bridge of your nose up toward your hairline. Massage your
eyebrows outwards toward your temples.
Drink less coffee and soda, and drink more water.
Invest in a comfortable computer chair with lower back support.
Hope this helps.
[edit on 2005/1/17 by wecomeinpeace]