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I was having a discussion with a friend about this. Usually, I understand that when traveling east, jet lag is worse than when traveling west.

However, when the time difference is (about) 12 hours, I posed that there would be no difference, as the directionality of the time shift makes no difference to the time of day at your destination (3 PM +/- 12 hrs is 3 AM, the only difference is the date).

My friend countered that it's not the time difference at the destination per sé that makes the difference, but the length of daylight during the flight. When traveling east, you'd fly into the sun, which would cause your perceived days to be compressed. When traveling west, your day would be extended. In their view, this made the difference.

I'd like scientifically sound proof of either one of these hypotheses, but personal experience is also much appreciated.

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    Voting to leave open. Scientific research may exist. It's not necessarily too opinion based just because it includes the word "worst". – Fiksdal Sep 14 '16 at 17:25
  • The argument about "daylight during the flight" is clearly wrong. On pretty much all long haul flights they close the shades right after departure and open them only just before landing so people can sleep or watch movies at their own pace. There is no way to tell whether you are flying east or west other than staring at the map. – Hilmar Dec 8 '16 at 12:59
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The widely publicized paper Resynchronization of circadian oscillators and the east-west asymmetry of jet-lag from the journal Chaos presents a mathematical model demonstrating that the brain's "oscillatory circadian pacemaker cells" have a more difficult time adjusting after an eastward flight compared to a westward flight crossing the identical number of timezones.

The New York Times article about it is more accessible; in short ,

it would take you about eight days to recover from a westward trip across nine time zones, if you did nothing to fight it. But if you cross the same number of time zones going east, recovery would take more than 13 days, according to the model. This recovery time is worse than if you flew smack across the globe, crossing 12 time zones, which is about the distance from New York to Japan.

The lead author explains the physiology in a Travel + Leisure writeup:

“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.… This is what causes you to experience more severe jet lag.”

Now, this paper has not yet been followed up with empiral observations on the matter. But another highly publicized paper, Chronic Jet-Lag Increases Mortality in Aged Mice (Curr Biol. 2006 Nov 7; 16(21): R914–R916.) similarly found that

aged mice were significantly affected by light schedule changes…. At the end of the 8 week period of light schedule rotations there was 47% survival in animals whose light cycle was advanced each week, 68% in those experiencing delays of the light cycle and 83% in unshifted aged mice…. Importantly, chronic stress was not implicated in this phenomenon…. To determine whether the effects of phase advances on mortality might be related to the duration between schedule changes, mice were shifted more rapidly, every 4 days. On this schedule, advancers died faster than with weekly shifts…. Delayers fared much better than advancers….

As you may have learned from high school health class, the natural human circadian rhythm is about 24½ hours, so our body resets its "internal clock" by exposure to sunlight, so neither advancing nor delaying the cycle will be pleasant. But at least for older mice, the former is far worse for them than the latter, so you'll need to do more preparation like sleep training or have more recovery for eastbound trips than for westbound trips.

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    Very interesting! The NYT article also states the following: Confusing? The model shows that your body is confused, too, as your cells try to adjust to new light cues in different places. It also shows that a trip less than 12 hours going east is going to feel worse than the same time going west. From this, I'd conclude that for 12 hours of time difference, there is no difference in traveling east or west. I'll check out the actual paper later. – Marten Jacobs Sep 15 '16 at 4:54
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    It's an interesting paper, but it doesn't answer the question at all, because Lu et al. don't distinguish between "12 hours eastward" and "12 hours westward" in their model. Look at Figures 2, 3 and 4, for example: there is a single line for the 12-hour difference, annotated as "12 E/W" or "12h east/westward". It's an implicit assumption of the paper that there's no difference between eastward and westward in the 12-hour case, and the model they present is incapable of demonstrating any such difference if it does exist. – Pont Dec 8 '16 at 9:15
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    This doesn't answer the question at all, since OP is specifically interested in a 12-hour lag. – JonathanReez♦ Dec 8 '16 at 11:57
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The direction you travel has no bearing on the severity of jetlag. Jetlag is caused by your body adjusting to the time difference. Direction only contributes to the length of time you are effected.

The difference between eastbound and westbound is more a matter of time of day you arrive. Eastbound long haul flights tend to leave late and arrive early. Combining tiredness from flying with the need to stay awake a long time usually results in day sleeping and prolonging the adaption.

West bound flights go with the sun, so when you arrive local night time is not far away, allowing you to get on local day / night cycle quicker.

Based in numerous east coast USA to SE Asia flights (basically 12 hours time difference) I prefer going west to arrive Bangkok at midnight and sleep. It is tough staying awake after arriving in the am on eastbound flights via Europe.

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  • OK, so actually, you'd prefer any flight that leaves early and arrives late, regardless of the direction of travel? – Marten Jacobs Sep 14 '16 at 8:26
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    @martenjacobs - Yes in general arriving late and going to bed based on destination time not body clock helps speed up your body's adjustment to the new daylight darkness schedule. – user13044 Sep 14 '16 at 10:03
  • Tom is right. Note that depending on your origin and destination, it may be difficult or impossible to find out such flights (for example, all flights between US West and AMS arrive there early morning or afternoon - I was unable to find a flight which would arrive late without making ridiculous itineraries) – George Y. Sep 14 '16 at 16:00
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Ending up somewhere west of home (even if you flew east to get there) means that you need to stay up later in order to go to bed at an appropriate local time, and you may wake "too early" for where you are. Many people can stay up 3 or 4 hours late once without feeling too awful, and can happily "sleep in" the next morning. (And if it's a business trip, you can go to bed at 7pm local and no-one will even know.) Needing to stay up 8 or 10 hours late would not be perceived as easy.

If you end up east of home, you need to get up earlier, which most people find difficult, and a 3 or 4 hour shift may be very hard, never mind an 8 or 10 one. Again this has nothing to do with the travel process that got you there.

This summer a young adult of mine spent 24+ hours travelling to somewhere 12 hours different (and back) 4 times. All the options were covered - out west, home east, then out east, home west. The report I got was that it made no difference what direction the plane flew and even whether it was light or dark outside the plane or a transit airport. It was just very difficult. FWIW.

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From personal experience, I find it easier to have a very long day (like 24 hours sunshine); I'll obviously be very tired, go to bed early, and wake up and everything is fine. The other direction, where 6 hours are cut out of a day and 6 hours cut out of a night, gets my timing totally confused.

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