Call it the “good” jet lag: On an east-to-west flight that touches down just before dusk, sleep finds you early and you wake up—often naturally—early enough to take in a full day somewhere new. “Generally speaking, it is easier to fly east to west,” says W. Christopher Winter, M.D., a sleep expert and author of The Sleep Solution. When you fly east, there’s less “sleep pressure,” adds Charles A. Czeisler, Ph.D., M.D., chief of sleep and circadian disorders at Brigham and Women’s Hospital in Boston, Massachusetts. Furthermore, species that are active during the day, as humans, feel a rush of energy just before dusk, he says. (That energy was likely helpful during evolution because, since we couldn’t see anything at night, we could scurry before sundown.). Westward locales could also fit our circadian rhythms more naturally: The human internal body clock is about 24 hours long. But most of us (three out of four people), Czeisler says, have cycles that run a few minutes longer than that, meaning we’re slightly more oriented toward being night owls. (If your clock was running a little bit faster, you’d be more of a morning person.)
Of course, these explanations don’t always hold for long-haul travel. At some point—especially as you get close to the 12-hour-time-difference mark (Beijing from the East Coast)—the body is more easily confused, says Winter. Switching hemispheres, which usually comes with stark seasonal and light changes, too, can throw the body for a loop as sunlight is the most powerful synchronizer of the body’s internal clock (which is why seeking sunlight out on a trip is just one way to help your body adapt). For these kinds of long-haul trips, making changes before travel using apps like Timeshifter can ease the transition, too.
But is it even easier to adjust back to your “home” time zone? There’s some truth to the idea that no matter the direction you fly, arriving home is easier on the body. Sometimes, for example, we don’t travel long enough to “adjust” to a foreign time zone, which can make our travels more difficult no matter the direction, but perhaps our returns a little smoother. (In the case of a short business trip overseas, for example, not adusting can even be beneficial, helping you fall back into your routine more seamlessly when you get home.)
Linger long enough to adjust to a vacation time zone? It isn’t necessarily a sentence for jet lag upon your return home, though it will take time—roughly, the “one day per one-time zone traveled” rule holds as a good measure of how long it takes the body to adapt, Winter notes. But the familiarity of your own environment and routine can often help you fall back into life at home, he says. “It’s a rhythm your brain has known for a very long time.”
