Gut Check looks at health claims made by studies, newsmakers, or conventional wisdom. We ask: Should you believe this?

The claim:

Jet lag is worse and longer-lasting when you travel east than west.

Tell me more:

This is such a common experience that we’ll mention only this study and this one that confirm you’re not imagining it. If you started in Miami, you feel much worse in Paris (six hours later than your biological clock insists) than in Honolulu (six hours earlier).

In the current issue of the journal Chaos, physicists at the University of Maryland present a new mathematical model of the oscillations of the brain’s pacemaker cells to explain why eastward jet lag is worse than the westward kind. They argue that eastward and westward trips “lie on opposite sides of the stable manifold of the saddle point” — a complicated way of saying that they find the answer in the physics of cells responding differently to the time change and acting upon each other.

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Really?

Experts in jet lag don’t think it’s necessary to invoke the physics of oscillators to understand the east-west difference.

Instead, the basic problem is that, traveling east, local bedtime comes earlier than at your origin (11 p.m. in Paris = only 5 p.m. in Miami), “and it’s more difficult to go to sleep earlier than you usually do, rather than later,” said Dr. Charles Czeisler of Brigham and Women’s Hospital, a leading expert on circadian rhythms.

In contrast, when you travel west it’s easier to stay up later, especially when surrounded by light cues and other signals (meals, activities, non-sleepy locals) that say, “Stay up, the night is young!”

Another factor: circadian rhythms. Our daily rhythms are such that we get a surge of energy in the evening, Czeisler explained — meaning our Miami-to-Paris traveler is getting a second wind just before Parisian bedtime, making it that much harder to wind down and catch some Z’s.

Our internal clock also produces a drive for sleep just before daybreak — again, “daybreak” meaning when the circadian clock thinks it’s near dawn. For the Miamian in Paris, that sense of exhaustion hits around lunchtime local time, and she struggles to keep her eyes open just when environmental signals (amount of daylight, meals, activity) say it’s the middle of the day.

Circadian rhythms do adjust. They have to: Their period is about 20 minutes longer than 24 hours, and so must be regularly reset by day-night signals or within a couple of weeks we’d be totally out of sync with the sun. But that adjustment — starting in the body’s master clock, a structure at the base of the brain called the suprachiasmatic nucleus—  is easier in one direction than the other, studies of lab animals have shown. Delaying the circadian clock (as when our Miamian’s biological noon must become 6 a.m. in Honolulu) is easier than advancing it, said biologist Horatio de la Iglesia of the University of Washington, “though we’re not sure why.”

What scientists do know is that a big reason jet lag feels so terrible is that the body has numerous internal clocks that control not just sleeping and waking but also processes such as the production and decay of liver enzymes and the activation of genes. “Going east, you get morning light at midnight your old time, so you’re getting light exposure for many hours before your normal wake time,” said Czeisler. Even if that message gets the body’s sleep-wake cycle entrained to the new time in a few days, the other clocks are harder to drag along and take longer to synchronize with the light-dark cycle, leaving them out of sync with one another. Result: sluggishness, feeling exhausted at inopportune times, even depression and cognitive fogginess.

The verdict:

Traveling several time zones to the east causes worse jet lag than flying the same number of time zones west, and although the precise mechanism isn’t known, it probably reflects the greater difficulty of advancing rather than delaying the body’s internal clock.

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