DO YOU feel tired most of the time? Dr. Frank S. Caprio, a prominent psychiatrist, writes: "Nervous fatigue is so common today that it has been referred to as the Great American Disease. Most patients who consult a doctor complain of chronic tiredness."

So great is the problem of always felling tired that some years ago Science Digest observed: "Every year doctors prescribe at least 3500 tons of amphetamine stimulants just to help their patients get through the day."

If you are one of those who are "tired all the time,' do you know the reason? What causes fatigue?

The diffuculty of answering this question may surprise you. The World Book Encyclopedia notes: "Doctors do not know exactly what causes fatigue. They do not know why a person feels tired after muscular exertion or mental effort."

Yet the answer may seem quite obvious to you. It is commonly thought that when a person works, he uses up energy, and waste products, such as lactic acid, accumulate in his blood. As a result, he becomes tired. Yet the matter is not all that simple, although there is certain truth in the above view.

That waste materials in the blood apparently are a factor in producing fatigue is shown by the fact that injections of blood of a fatigued animal into one that is rested will produce fatigue in the rested animal. Yet The Encyclopedia Americana comments:

"Under norma conditions . . . the muscle is kept supplied with sufficient nutrient material and the waste products are reconverted to supply new energy so long as the blood supply of the muscle is intact. It is therefore unlikely that these chemical changes are of critical significance in normal fatigue, except possibly in very heavy muscular work. For normal kinds of sedentary work, chemical changes in the muscles play a minor part."

Common experience also runs counter to the view that fatigue is due simply to expenditure of energy and chemical changes in the muscles. Consider, for example, the worker who feels fatigue after hours of work, but then his fatigue suddenly leaves him. You probably have had that experience.

Perhaps you can remember being tired after hours of work. But then you were invited to do something that you really enjoyed, such as play a game or go for a hike in the woods with friends. Even though this required perhaps more energy than the work you were doing, almost at once your tiredness vanished!

You probably have had similar experiences that make the question of fatigue baffling. For example, you may have discovered that you are less tired when doing work lyou enjoy than when doing even easier work that you do not enjoy. Many persons, in fact, get tired while hardly exerting themselves at all. Even the thought of having to do certain things makes some persons tired! Such true-life experiences causes the journal today's Education to conclude: " There is something wrong with the common assumption about what causes fatigue or even about what fatigue is."

The book Fatigue and Impairment in Man, after presenting extensive research on the subject, explained: "Fatigue . . . bears no consistent relation to expenditure of physical energy."

If feelings of fatigue are not due simply to the using up of energy, what causes that tired feeling? Various factors are known to be involved. By NATHANIEL HAGOOD