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… 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.