The TRAVELER'S JOURNAL: July twenty-sixth, shedding light on jet lag.

Forget melatonin and faddish diets. When it comes to avoiding jet lag, just turn on the lights.

Harvard sleep researchers blame jet lag on "external and internal desynchronization." The theory claims long distance flights force the body to cope with sudden shifts from day to night, cramped in desert dry planes, coaxed to sleep at odd times. Light and dark cue the brain to reset its internal clock each day. Strong doses of light can advance that clock.

The cure's first step is to determine whether you're a "morning lark" or "night owl." Then counter the effects of jet lag with massive doses of light at the proper time. Calculating when and how much depends on the direction you're flying, how many time zones you'll cross and how much sleep you'll lose. Light received before 5 a.m. home time shifts the traveler's time clock westbound. Later light shifts it eastbound. Travelers can get the necessary light in artificial settings with a $300 "light visor." A cheaper alternative is to simply take a walk outside in the daylight, once you land of course.

Choice of flights is also important. Some departure times are better for different types of travelers. Morning folks who adjust rapidly to new time zones can take advantage of daytime flights from the U.S. to Europe. Night owls may find the last flight at night lets them sleep on the plane. Business travelers on short trips to Asia or Europe may do better not adjusting their biological clocks at all but stay on their home time zone. The light cure doesn't claim to eliminate jet lag, only to make long distance transitions much easier.

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