Work-related Headaches’ causes: light
Too much, or too little light can cause headaches. Factors such a the contrast of computer screens and the glare of sunlight are covered elsewhere on this book.

Fluorescent Lighting.
Although fluorescent tubes appear to be giving off a steady shadowless light, in reality they flicker one hundred times per second. Because the retina smooths out these little bursts of light, normally we're not consciously aware of the flickering. However, this flickering still has a very special and very odd effect. When you switch your gaze from one point to another, the eyes move in a smooth line; however, if the fluorescent light is in its 'off phase at the time you are about to look at another point in the room, your eyes can't see it, and will overshoot it slightly. When the light comes on a hundredth of a second later, your brain realises that it's overshot the mark and starts moving your gaze back again until it gets to the right position.
Therefore, under fluorescent lights your gaze is constantly overshooting and needs constantly to be corrected. Making these extra corrections is tiring on the eyes, and is one of the reasons why so many people find working under fluorescent lights irritating and tension-making.
There is a very quick way to stop this happening - double the frequency of the current in the lights. Then the fluorescent tubes flicker at two hundred cycles per second, which is too fast to allow the eye overshoot to occur. Working under fluorescent lights like this is much more restful, but lamps like this are more expensive than the ordinary sort.
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Work-related Headaches