WHY do your eyes hurt when you are tired?

Couch potatoes are busier than you might imagine. While lying on the sofa perusing a comic book or studying the impact of televion violence on children by viewing bugs Bunny cartoons, they are actually exercising what ophthalmologist James P McCulley calls 'among the most active muscles in the body'.

Actually, your eyes contains three sets of muscle groups:

• Each eye has six extraocular muscles attached to the outside of the eyeball, which turn the eyes in all directions. The extraocular muscles must coordinate their movements so that both eyes look in the same direction at the same time. 

• The sphincter and dilatory muscles open or close the pupils, defining how much light is allowed into the eye.

• The ciliary muscles attach to the lens inside the eye. When these muscles contract or relax, they change the shape of the lens, altering its focus.

Concentrated reading or close work provides a workout for these muscle groups strenuous enough to make your aerobics instructor proud. Unfortunately, as in all aerobic programs, the saying 'no pain, no gain' applies, as optometrist Steven Mintz explains:

'The human eye is designed so that, if perfectly formed, it will form a clear image on the retina (at the back of the eye) of any distant object without having to use any ot the muscles. In order to see closer objects clearly, however, each set of muscles has to work. The extraocular muscles must turn each eye inwards: the sphincter muscles must turn each eye inward; the sphincter muscles must work to make the pupil smaller; and the ciliary muscles must contract to allow the lens to change to a shape that will produce a clearer image.

This minimal muscular effort is significant in itself. However, no human eye is perfectly formed and these imperfections will increase the amount of effort required. Virtually every person, as [he or she] approaches or passes the age of 40, suffers from a stiffening of the lens inside the eye, which forces those ciliary muscles to work even harder.'

Eye specialist are finding that people who work on computers experience descreased blinking. This is one reason most consultant recommend stepping away from the computer at least once an hour. While most of us are more than happy to rest our muscles after doing a few pusp-ups, we expose our eyes to a marathon just about every day.