Has anyone come up with a surefire way to cope with jet lag and traveler's tummy?

Catch up on your sleep ahead of time, advises Dr. O.H. Rundell, adjunct assistant professor of psychiatry and behavioral sciences of the Oklahoma University Health Sciences Center in Oklahoma City."People often leave in a sleep-deprived condition. They may stay up late packing or get up at an early hour the day they're leaving," he says.

Instead, get into your new time schedule two or three days ahead, he advises. "If you're going to California from Kansas, go to bed at 1 a.m. if you usually go to bed at 11 p.m. If you're going to New York from Kansas, go to bed an hour earlier to get your body on New York time."

On the other hand, if you're only going to be away for a few days, adjust your activities to fit the schedule you're used to, he says. "If you're from a Central Time Zone and you function best at 10 a.m., you'll set your meeting for 11 a.m. in New York or 8 a.m. in California, taking into account the time differences."

Eastbound is tougher than westbound, says Rundell. That's because you're moving forward in time and against your biological rhythm.

- It's not what you eat but when you go to the bathroom that upsets your regularity on the road, says Dr. Robert A. Rankin, associate professor of medicine for digestive diseases and nutrition at Oklahoma Memorial Hospital. "If you get up every morning at 7 a.m. and go to the bathroom, your body's going to be used to that. If you change that routine to another time, you may suffer constipation."

He suggests trying to stay on your regular eating schedule. Take an over-the-counter laxative if needed, and don't wait several days to do it. Snack on fruits and vegetables. Restrict your intake of rich foods and drink to avoid heartburn. And if traveling to Mexico or any area with poor drinking water, bring along a diarrhea remedy.

- Despite the growing popularity of cruise vacations, there are still those who suffer from seasickness, which is an extreme form of motion sickness.

"If you are prone to motion sickness when on a plane, train or car trip, then stay away from long vacation cruises," advises Dr. Christopher Linstrom, assistant director of otology (specialist in inner ear disorders) at the New York Eye and Ear Infirmary.

Linstrom suggests that if you are unsure of your susceptibility, test yourself by taking a short boat ride.

"Those who are likely to become ill feel it quickly - within six hours," he explains. "In my experience, it's rare for a patient to be fine for three or four days and then suddenly get seasick."

Linstrom says seasickness occurs when the body loses its sense of balance and the brain cannot synchronize incoming stimuli from the inner ear, eyes and joints.

Most of his patients, he adds, are post-cruise line passengers.

"After being on a cruise or deprived of land for some time, many patients when back on land complain of feeling as if the boat is still rocking. They cannot regain their balance.

"Most of them will get better with time and by doing some gentle head motion exercises."

- Traveling Healthy is a new newsletter about staying well and dealing with health problems while traveling.

For example, you can find out if the flu will ground your flight plans, what immunizations you may need before you start out, where in the world outbreaks of various diseases are showing up, and how to survive a hotel fire.

Executive editor is a doctor, Karl Neumann. Subscription is $24 a year, at 108-48 70th Road, Forest Hills, N.Y. 11375. --

So you decided to go to the Southwest for some winter sunshine and rest. But on arrival you find yourself sneezing, because the long weed season in that part of the country begins in February.

Allerest is offering consumers a free allergy calendar, a wheel-type of calculator to help you figure out when trees, weeds and grasses are at their height in various parts of the country. For the calendar and other information about allergies and travel, call the company's allergy information center at 1 (800) 727-5400.