Jet age travel works wonders for business but wreaks havoc on the body. A Washington-to-London flight can result in exhaustion, sleeplessness and a severe headache -- otherwise known as jet lag.

As any traveler knows, crossing time zones by bus, train or ocean liner won't cause jet lag. It only happens when you rapidly cross several time zones and throw your internal, biological clock out of whack.

A group of brain cells acts like a pacemaker to govern the body's internal clock, said Dr. Richard Coleman, a clinical psychologist at Stanford University Medical School and author of the book "Wide Awake at 3 a.m. -- By Choice or By Chance?" The cells determine alertness, sleepiness and biological functions such as the movement of bowels or release of hormones.

The circadian cycle (from the Latin for "around day") follows a 25-hour clock but easily adjusts to a 24-hour schedule through external cues such as daylight and mealtimes.

"If I suddenly transport you to England, you're going to find that things are out of sync," said Coleman. "When you go to bed at 11 p.m. London time, it will be 6 p.m. Washington time."

How can a traveler be alert for a 9 a.m. appointment in London when his or her body is still in the middle of a Washington night?

While President Johnson was in Saigon during the Vietnam War, he dealt with the problem by holding meetings at 9 a.m. Washington time, Coleman said, even though it was 10 p.m. He ate and slept on the same schedule as if he had never left the White House, ignoring the time around him.

Most people, though, cannot set their own schedules when they travel. If people could reset their internal clock in the new time zone as easily as they reset their wristwatch, jet lag would not exist. That being impossible, current approaches focus on manipulating one's schedule in advance to adjust to the new time zone. However, flight cancellations or sudden trips often interfere with the advance preparation required.

Traveling east is more difficult to adjust to than traveling west, say specialists in jet lag. A person has to go to bed earlier each day before the trip, which is difficult because of the body's natural tendency to stay up later, following the rhythm of a 25-hour clock. "You can only shift back around an hour at a time," Coleman said.

Once you've arrived at your destination, such as Europe, it also helps to try to adapt to the new time zone immediately. Stay up and delay sleep as long as possible. "If you make an immediate adaptation to the environment, you will adapt much more quickly to the circadian changes involved," said Dr. Michael Thorpy, director of the Sleep-Wake Disorders Center at Montefiore Medical Center in New York.

Short-acting sleeping pills may also be useful. They can increase the quantity and quality of the first night's sleep after travel and improve daytime alertness as well, said Thorpy. Research shows that a short-acting medication is helpful for several nights after an eastward flight, whereas for a westward trip, it may only be necessary for one or two nights, he said.

A westward trip, such as returning home from Europe, is easier. If a person is traveling west, he or she should start going to bed later and later each night before the trip. It is possible to push yourself ahead three hours in one day, Coleman said.

Some people believe that changing the diet can also help. Dr. Charles Ehret, senior scientist in the division of biological and medical research at the Argonne National Laboratory in Illinois, puts forth this theory in his book, "Overcoming Jet Lag" (Berkley, $5.95). He recommends a four-day feast/fast diet before east or westbound flights. On the feast day, he suggests eating several hundred calories more than usual; then several hundred calories fewer on the fast day.

The fast day preceding the trip is then followed by a high-protein breakfast at the destination point. This diet is based on the differing effects protein, carbohydrates and caffeine have on the brain cells that govern the body's clock.

"High-protein foods such as fish, fowl, eggs, meat, dairy products and beans, stimulate the adrenalin pathway and give you up to five hours' worth of long-lasting energy," Ehret says in his book. "In contrast, a meal consisting principally of high-carbohydrate foods, such as pasta, salad, fruit and rich desserts, gives you a surge of energy for up to an hour but then actually encourages you . . . to go to sleep."

Ehret's jet-lag diet has never been scientifically tested and is treated with skepticism by some experts. "It requires a superhuman effort to keep to the plan," Thorpy said. "Also, I'm not sure that there is really good evidence that even if one does adhere to it, that it deals effectively with the problem."

But many people -- military and industrial personnel as well as travelers -- say they have used the diet and find it effective.

Dr. Eugene D. Erman, a retired surgeon and frequent traveler, flew to Israel this past spring and tried the diet. He and his wife used the feast/fast approach and did not drink any caffeine for four days before the flight. "When we got on the plane in Los Angeles, we set our watches for the time of the point of our arrival," Erman said. "We took a late afternoon flight, and when everybody was being served dinner, we had arranged to have breakfast. We drank our first coffee then."

They continued to eat their meals on the plane as if they were already in Israel. "When we arrived, it was about bedtime; we had had breakfast, lunch and dinner, and we were ready to go to bed," he said. "We woke up the next morning tuned in."