Bad lighting can cause eye strain and increase fatigue, as can light that is too bright, or light that shines directly into your eyes. Fluorescent lighting can also be tiring. What you may not appreciate is that the quality of light may also be important.
Try experimenting with working by a window or using full spectrum bulbs in your desk lamp. You will probably find that this improves the quality of your working environment.
Create a mental image of a pleasant and relaxing place in your mind. Involve all your senses in the imagery: see the place, hear the sounds, smell the aromas, feel the temperature and the movement of the wind. Enjoy the location in your mind.