It's a misnomer, of course. You don't get jet lag from jets. Not from the engines, not from the stream.

You don't even have to travel to get it.

It was simply that no one noticed it very much, really, until all those zombies started pouring out of all those jetliners. Especially eastbound.

In fact, jet lag is more accurately known as circadian desynchronization -- and more simply known as getting your body clock messed up.

Knowledge of the physiological basis of sleep and sleep disorders has burgeoned in the last decade or so, and enough studies have been done to permit some conclusions about jet lag -- and its care and feeding.

The human body is an extraordinarily complex operation run by an even more complex computer, the brain.

Physiological operations are generally cyclical, perhaps even more so than has yet been established, some experts feel. And some of the body's biological rhythms have been measured and confirmed. (These have nothing to do with the so-called biorhythms which purport to predict "up" days and "down" days on the basis of one's birthday.)

Scientists call the body cycles that are roughly geared to a 24-hour day, circadian rhythms. Through their operation such functions as heartbeat, body temperature, perhaps even hunger, as well as the functions of the endocrine glands, the cardiovascular system and the nervous system are regulated.

There is time for your steroid levels to be high and a time for them to be low. There is a time for you to run a seven-minute mile and a time for you to sack out . . . There is a time to sleep and a time to wake . . .

When you go from day shift to night shift and back to day shift on your job, or when you fly into the sunset, you are inflicting strains on your body clock that are enough to pop any self-respecting mainspring.

The body's clock, however, is extraordinarily resilient. In general, it will adjust. It's your clock, after all, and really quite accommodating to your needs. Custom-built, you might say.Given half a chance, it might adjust even before you jolt it by a trip into a piece of yesterday or tomorrow.

But the bottom line is this: When you shock your body clock, throw a monkey wrench into those delicately balanced rhythms, you're going to know about it.

And if you've ever wandered around the streets of London around 3 a.m. with nary an open pub, not even a chimney sweep to convince you you're not alone in the world, well, then, you see what I mean.

Airline passengers, of course, aren't the only victims.

More and more attention is being paid to the repeated circadian disruptions suffered by pilots and other aircraft personnel, disruptions which could have ominous ramifications. Shift workers like air traffic controllers and nuclear power plant workers are also at risk.

Rep. Barry Goldwater Jr. (R-Calif.) who is a pilot as well as legislator, has been concerned for some time about the possible relationship between air accidents and possible body clock-zonked pilots.

Joe Tymczyszyn, Goldwater's staff assistant on the Science and Technology Committee, has researched jet lag in all its ramifications. He is not a scientist, but has done a good bit of research, using himself as a guinea pig, and may be one of the top jet lag savants in the world.

In part as a result of Goldwater's efforts, NASA is conducting a three-year study of the problem and will eventually publish a pamphlet useful to laymen as well as to pilots. They will also do a field study on pilots at work and in NASA simulators where experimental swings in shifts would have only experimental fallout.

Meanwhile, Tymczyszyn (pronounced, roughly, Timshizzen) has some suggestions of his own for averting jet lag -- or at least lessening its punch.

First, before a trip begins: Start as many days ahead as there will be hours of difference in time. (For example, if you're going to London where the time difference is five hours, start five days ahead.) Each night go to bed one hour earlier and get up one hour earlier. Body clocks can be reset, just not all at once. This way, by the time you land at Heathrow, you'll be just about on London time.

During the trip:

If it's a night flight, it's important to try to sleep, even if it's only three or four hours. Says Tymczyszyn, "a five-hour disruption is more severe than only getting four hours of sleep one night."

"Lay off alcohol." Hard liquor may put you to sleep, he says, "but it's the wrong kind of sleep and you wake up a wreck." On the other hand, a glass or two of wine is helpful.

"Lay off caffeine." Not only will coffee interfere with sleep, but there are good studies that suggest that caffeine makes it harder for the body to start readjusting to a new time.

Eat lightly at the beginning of the trip and have a good meal at the end. "If the body has to work on digesting a meal for four hours or so, it won't have the flexibility to start shifting." What Tymczyszyn does (stewardesses must just adore him is this: "If it looks like a meal that will keep, I just ask the stewardess to put it away until an hour before landing . . ."

Drink a lot of non-alcoholic liquids."The air in a jetliner is drier than the Sahara desert," he says. Dehydration isn't actually related to jet lag at all -- just altitude.

Walk around every once in a while, just to keep things moving.

Studies have shown that traveling west to east is a lot easier on the circadian rhythms. Europeans have better vacations in the United States than Americans in Europe, says Tymczyszyn, not just because of the relative strength of the dollar. "It's because on the second day they're raring to go," he says. This is true because the body adjusts more easily to stretched-out days than foreshortened ones. You can almost always stay up until the wee hours once in a while if you need to, he notes, "but did you ever try to go to sleep four hours early?"

Airlines themselves are becoming more sensitive to the problem for both passengers and personnel, although no airline has yet tried serving the snack after takeoff and the dinner before landing.

However, Scandinavian Airlines (SAS) has begun a successful promotion of sitting exercises for passengers on long trips. An SAS spokesman in New York, S. Ralph Cohen, says they help with such side effects as backache and grogginess. They were developed by Swedish fitness expert Folke Mossfeldt.

Among them are these:

Warm up by "jogging on the spot": "Simply raise heels alternately as high as possible. At the same time raise arms in bent position and rock rhythmically forward and back, as when walking. Continue for three minutes."

"Sit with elbows on knees, bending forward with your whole weight pressed down on your knees. Lift up on toes, with heels as high as possible. Drop heels and lift your toes. Repeat 30 times. This improves blood circulation to legs when sitting."

Shoulder rolling "stimulates joints and relaxes shoulder muscles."

Head turning and nodding.

Copies of the SAS exercises, Cohen said, were sent to the Iranian hostages.

As another measure of their success, he tells of a woman he met at a party who raved on and on about how wonderful and helpful the exercises had been. "Unfortunately," Cohen said ruefully, "as far as I know, she's never travelled on SAS."