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Jet lag, explained.

July 12, 2016

Even if you haven’t experienced severe jet lag after an eastward flight, you’ve probably heard this to be true: Recovery from jet lag is harder when you’re headed east than west.

In a study published in the journal Chaos, researchers say they have now identified the physiological reason travelers have more difficulty adjusting to the time change when flying east rather than west.

“Basically, what we show is this eastward-westward asymmetry ... comes from the fact that your [circadian rhythms] have a natural period that is a little longer than 24 hours,” Associate Professor Michelle Girvan told Travel + Leisure.

So instead of being exactly 24 hours, your internal clock could be, for example, 23.5 hours or 24.5 hours.

Girvan and her co-authors chose to study the individual pacemaker cells, or neuronal oscillators (stay with me), that dictate our natural circadian rhythms. They found that if a person's internal clock has an additional 30 minutes, it could translate to days of jet lag when rapidly crossing time zones.

“You expect to advance your internal clock if you travel east and backward if you travel west,” Girvan added. "However, if you travel a large number of time zones eastward, your internal clock doesn’t phase advance like you would expect. Instead, it phase delays.”

For non-scientists, “phase advance” is your bedtime and wake-up time moving earlier in the day, and “phase delay” is your bedtime and wake-up time moving later.

“This is what causes you to experience more severe jet lag,” said Girvan.

As a general rule of thumb, the team discovered that eastward travel across three time zones takes a little more than four days for complete recovery. A trip westward across the same number of time zones requires a bit less than three days. For travel eastward across nine time zones, complete recovery takes almost two full weeks. A traveler can recover from a trip westward across nine time zones, on the other hand, in less than eight days.

Only when a full 12 time zones have been crossed, in either direction, will the recovery time be nearly equivalent (about 10 days).

The takeaway: If you feel more jet lagged after traveling eastward, the math is on your side.

To combat jet lag, Girvan says you should be conscientious about when you’re exposed to light, even the artificial glow from your computer screen.

“You can try to acclimate to a new time zone ahead of your travels by using artificial light in a [simulated light environment]," Girvan suggests. She also notes that while the research only examined neuronal oscillators, many other cells have circadian rhythms controlled by forces other than the rising and setting of the sun.

“You can imagine what you eat influences the schedule of these things ... it's another level of complexity,” she said.

Melanie Lieberman is the Assistant Digital Editor at Travel + Leisure. Follow her on Twitter and Instagram at @melanietaryn

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