With that in mind, here are the general guidelines: If you are traveling east, you must expose yourself to light early, advancing your body clock so that it will be in sync with the new time zone. Conversely, if traveling west, you should expose yourself to light at dusk and the early part of the evening, delaying your body clock so that it will be in sync with the new time zone.

This may be best understood with an example. Let’s say that at 7 p.m. you board a plane in New York that is scheduled to arrive in London at 7 a.m. local time (when it’s 2 a.m. in New York). You’re traveling east, which means you need to advance your internal clock toward London time. To do that, avoid any kind of light during the flight because the exposure will delay your body clock rather than advance it. An obvious (albeit odd) way to accomplish this is to wear sunglasses in the plane. That’s what Lockley and his colleagues do despite the fact that they are flying at night.

“People think you’re a rock star,” he said.

Typically, when travelers arrive in London at 7 a.m. they attempt to get on the new time zone right away.

“Which is exactly the wrong thing,” Lockley said, because your internal clock is still set to New York time, and trying to adjust too quickly will only exhaust you. What you need to do is to ease yourself into the new time zone by consciously manipulating your exposure to light. So keep those sunglasses on.

“I’m the only person wearing sunglasses at Heathrow,” said Lockley, who, in the London example, would recommend wearing sunglasses for the entire flight, and once off the plane, until 11 a.m. London time (6 a.m. New York time).

Throughout the rest of the day, seeing light will help you to be more alert and to reset your internal clock to local London time. (For those who want to get granular, the new book Sleep: A Very Short Introduction, which Lockley co-authored, provides details about which hours of the day exposing yourself to light or darkness will be most beneficial to overcoming jet lag.)

If you are able to sleep during the flight, even better. Astronauts and mission-control personnel have used eye masks, earplugs and sleep aids like Ambien to help them doze, Johnston said. But he cautioned travelers who want to take a sleeping pill to check with their doctor first and to avoid taking any medication with alcohol. Many airline passengers “just get drunk and pass out,” he said, underscoring that a hangover does nothing to alleviate jet lag.

Those who want to take synthetic melatonin because it might induce sleepiness during a flight should also consult a doctor first to find out if it is safe for them. Furthermore, as the Centers for Disease Control and Prevention caution, synthetic melatonin is not regulated by the Food and Drug Administration.

Now, if you were to take a morning flight instead of an evening flight to London from New York, you would want to expose yourself to light throughout the flight (no need for sunglasses), as well as when you land in London, soaking up as much sun as possible all day.

“You can have exactly the same trip but the advice is opposite depending on what time you’re taking the flight,” said Lockley, who has also used these principles to help racehorses acclimate to new time zones. “Once you understand the timing issue you can go through that process for any trip.”

•  Survive the first night by eating right and preparing the hotel room for a good night’s sleep. Whatever you do on your first day, remember that the things capable of upsetting your body when you’re at home can be even more troublesome when traveling. For instance, some of us know that alcohol may help when it comes to falling asleep but that it can interrupt later stages of sleep, which would only exacerbate jet lag. Large or spicy meals should also be avoided in the evening at your destination, Lockley advised, because the body is not as efficient at metabolizing food at that time.

At night (and for each night of your London trip) about an hour or so before bed, keep the lights in your room as dim as possible. Close blinds or curtains and cover any light from a clock, computer, television, even your smartphone, because light can make you more alert and reset your internal clock to the wrong time, making you think the day has begun.

More tips on improving sleep at home or on the road are available at Harvard University’s “healthy sleep” website, healthysleep.med.harvard.edu/healthy/getting/overcoming/tips. Just don’t log on before bedtime.