Fly from Washington to London, and you may find yourself pacing the streets while the city sleeps.
Fly from Washington to Australia, then to Singapore, Korea and Japan, and you may find your internal clock thrown completely out.
Although President Bush has jogged, played tennis and dined with dignitaries, it would be normal for someone on such a pace to suffer alternating bouts of fatigue, sleeplessness and confusion.
"You take it for granted that all our body is operating very productively and comfortably," said Dr. David Neubauer, a psychiatrist at the Francis Scott Key Medical Center. "But a great deal of it depends on the logic underlying our internal clock.
"Just about everything you can measure -- your blood pressure, your heart rate, your degree of alertness and the timing of your urination -- is really orchestrated with that clock. It's the reason we tend to go to the bathroom in the daytime and not in the night."
Jet lag and the president's grueling schedule could have produced enough fatigue to make him vulnerable to the illness that produced his sudden fainting spell yesterday, Dr. Neubauer said. But it is doubtful that jet lag alone -- in the absence of an infection -- was enough to put the president on the floor, he said.
There are steps people can take to prevent extreme jet lag, Dr. Neubauer said.
But for people following the president's itinerary, "it's impossible to do it," Dr. Neubauer said. Only time itself can help someone on that schedule. And the adjustment could take a week to 10 days.
Every year there's something new. Here is a list of the medical problems President Bush has had since he took office in 1989:
* Oct. 6, 1989: Undergoes minor surgery for the removal of a benign cyst from the middle finger of his right hand.
* April 12, 1990: During a routine medical test, it was discovered that Mr. Bush had glaucoma in his left eye. It was treated with eyedrops.
* May 4, 1991: Mr. Bush is hospitalized after experiencing shortness of breath and fatigue while jogging at Camp David. He was diagnosed with a condition known as atrial fibrillation, or a quickened and irregular heartbeat. On May 7, the White House physician announced that the heart condition was brought on by a hyperthyroid condition.
The president was treated with prescribed medications.