The symptoms of jet lag result from two things: the sleep deprivation and fatigue that come with long stints in a cramped seat, and the misalignment of your body clock with the local time zone, says physician Vivek Jain, medical director of the George Washington University Hospital Center for Sleep Disorders.

Jet lag’s severity depends on several factors, the most obvious being how many time zones you cross. But which direction you travel matters, too. Turns out, it’s not my imagination: Going east really does produce worse jet lag.

TRAVELING WEST IS EASIER

Your body clock has an innate tendency to run slightly longer than 24 hours, Jain says. Each morning, your body compensates for this slight discrepancy by contracting your internal clock to synchronize with the 24-hour sun cycle. When you travel west, you gain several hours, so your day is extended and your body gets the extra time it naturally wants. But when you travel east, your day is shortened; that makes it harder to adjust, Jain says, because your body has to cut its natural cycle even further.

Studies suggest that you can push your body clock back about two hours per day, meaning that you can adjust from Eastern time to Idaho time in a single day, but you can move your body clock forward (as when you travel from California to New York) only by about an hour to an hour and a half per day, Jain says.

So for easterners flying to see friends or family in California, where it’s three hours earlier, your body clock should be able to adjust in less than two days. But if you’ve got a vacation planned in Paris, where it’s six hours later than the East Coast, you’ll likely need three to five days to get in sync.

AVOID NIGHT FLIGHTS, ALCOHOL

• Time your travel well. Rest up the night before and book a daytime flight if you can. Red-eye flights compound jet lag by heaping sleep deprivation on top of your body-clock problems.

• Keep hydrated to avoid headaches and lightheadedness, and stay away from alcohol on the plane. Alcohol interferes with sleep.

• If you’re heading east, aim for an evening arrival. Have a quick dinner and go to bed.

• Before your trip, ease your transition to the new time zone by moving your bedtime. The American Academy of Sleep Medicine guidelines suggest shifting your sleep schedule an hour earlier each night, starting three days before you leave on an eastbound trip. If that’s impractical, Jain says, try for 15 minutes earlier each night and get as much early-morning sunlight as you can. Light is the most potent tool for adjusting your body clock: Think of it as a medication that wakes you up, he says. For a sleep adjustment chart, see the link at the end of this article.

• A walk might help, too, though its effects are much weaker than light’s.

• Melatonin, a hormone normally excreted during sleeping hours, can help shift your circadian clock by inducing drowsiness. Though Jain cautions that the studies on melatonin are mixed, a review of 10 studies by the Cochrane Collaboration, an independent research group, concluded that melatonin can be “remarkably effective” at reducing jet lag, and the American Academy of Sleep Medicine guidelines suggest that a dose of 0.5 to 5 milligrams of melatonin taken at bedtime may help you adjust when traveling east.