When an executive hits those "middle years" strange things often begin to happen. He's up near the top of the management ladder; his children are through school and probably raising families of their own; he can still put enough belt into his golf swing to whip some of the youngsters at the country club.

Everything should look rosy. But then he starts feeling tired and listless. His energy begins to ebb too fast each day. Few businessmen in such a position are ready to admit publicly that they're constantly feeling fatigued. But many physicians maintain that fatigue is the No. 1 complaint among middle-aged businessmen.

There is a chance, of course, that tiredness and listlessness may stem from some physical problem: So, obviously, you need to see a doctor to make sure that it isn't caused by anemia, diabetes, or heart trouble. But, say doctors, at least 80% of middle-aged businessmen who complain of fatigue can pass a physical with nothing more serious than an admonition to lose 10 lb. or a suggestion to get new reading glasses.

Most often, the root of the problem is something that may surprise you: It lies in a deep-seated, unrecognized boredom. Typically, the "tired" executive is as far up as he's going in his company or industry. Much of the old challenge is gone. He's working as hard as ever, maybe harder, but he gets a lot less stimulation from his job. The thought of eventual retirement becomes lodged firmly in the back of his mind about this time. That adds to frustration. Inevitably, he begins to slow down a bit physically.

At home, with the children gone, his own marriage often lags in stimulation. His social life has taken on a dull sameness. These are the things that add up to middle-age "executive fatigue" - the physicians' euphemism for boredom.

The problem, then, is what to do about it. Doctors recognize that the solutions they offer to the busy but frustrated executive who comes to them seeking diagnosis and relief from his fatigue all too often sound like a Boy Scout lecture.

Get a strong hobby interest and get regular daily exercise are often the nub of the prescription. The businessman will usually snort and say he is much too involved with his work to do that. But, say physicians, following such advice is about the only way to beat the boredom that's at the root of all the trouble. And enough doctors have seen enough good results among executives who have followed that prescription to be sure that there is value in the standard remedies.

But there's more to this line of advice: Take more vacation time, change your pace at the office, do more business travelling, find a new outlet for creative energy. High on the list also is: Don't take your work home, but - if you feel you must - then make it a minimum amount. This, say doctors, definitely will increase your efficiency in the office - especially if you are a top executive who shouldn't be worrying about routine details, anyway.

Getting enough sleep - and getting it without the aid of sleeping pills - is also most important. Fitful or insufficient sleep only adds to boredom and fatigue. At age 50 you probably need eight hours of sleep a day - and certainly no less than seven hours. You can't safely get by with five or six hours on week nights and then try to make it up by sleeping more on Sundays. If nothing else, your efficiency will be impaired.

You have to learn to set aside worries and stresses before you retire. It isn't always easy. Try a bottle of beer followed by a warm (not hot) shower at bedtime. Or unwind by taking an easy 15-minute walk outdoors.

Occasionally "executive fatigue" signals a serious emotional problem that requires some type of psychiatric care. In such cases a person will usually show additional symptoms: irritability, restlessness, too heavy drinking - and in the executive, anxiety about facing the job and a seeming inability to make sure decisions. Here an emotional check-up is needed.

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The Flexible "fixed-income"

Have you thought about the highly flexible "fixed-income" trust? Some advisers currently are recommending it. Here's the idea: Instead of the more usual trust that has income going to a man's wife for her lifetime, then to the children for a period of years, and finally outright to the children, the husband says to the trustee: "Pay a fixed sum to my wife each year - and if the income falls short of this liberal amount, then you may reach into the principal."

In effect, you tell the trustee to sell off some of the securities - if necessary - to obtain capital gains for your widow's use. If invading principal worries you, an adviser will likely point out that these days many persons, with... see: The Flexible "fixed-income"