What you have when you've been travelling through different TimeZones and your body's circadian rhythms are telling you to sleep (or not) when everyone else in this area is awake (or sleeping).

Can usually be cured by staying awake until a little before everyone else nearby is going to sleep, so you can roll over.

''Depends on the severity. Change more than 8 hours and you can count on days of JetLag unless you do a sensible amount of MelaTonin.''

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I have the distinct impression that one of the big contributors to JetLag isn't this timezone thing at all, but rather the fact that you have just spent 8+ hours packed like cattle in a badly-ventilated tin can. The worst JetLag I ever had was while flying from Amsterdam to Johannesburg, an 11 hour flight. The thing is... there is only 1 hour time difference between the two cities.

Of course, the airlines ''(Ah, you mean the OneWorldAlliance!)'' would like to have you believe that the real reason why you're feeling so bad is that it is 4 AM back home. Never mind that ''back home'', this never seemed to be such a problem. 

-- StephanHouben

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It is my experience from various trans-atlantic flights that going west causes a lot less jet lag than going east.  So time zones do matter in my experience. -- RuudDeRooij

Apparently

* 80% of people find it harder going Eastward and easier going Westward
** ''this is soooo weird''
*** Not at all. Research shows that when people are given no time feedback their bodies settle to a rhythm of about 28 hours, with some variation between individuals. (There are plausable arguments based on evolution to suggest why this is so, although they're not airtight.) When you go Westward the adjustment you have to make is to have some longer days to get into sync, and that's what your body is good at.
* 10% of people find it harder going Westward and easier going Eastward
* 10% of people find no difference.
The problems caused by the shift in body-clock is a documented effect. No doubt travelling first class makes the experience more pleasurable and reduces the fatigue, but JetLag is still there even when fully rested.

Having VeryGoodSeats ''does'' have a positive influence on this.

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I seem to agree with StephanHouben:

I recently flew from the east coast of the US (GMT-5) to Thailand (GMT+7), and experienced little jet lag.  I was there for 1 week.  Upon returning home, I experienced terrible jet lag.  The astute reader will notice that this is a time difference of 12 hours, so it is irrelevant which direction I travelled: it doesn't matter if I gained 12 hours or lost 12 hours, it would still be the same time of day in Thailand, and the sun would still be in the same place in the sky.

I propose the reason for the lagging of jet on my return home is that while in Thailand, despite it being January, the weather was perfect and I was outside in the blazing sun (high in the 80's F) and having fun for a good 12 hours a day.  I got little sleep at night, but I was not tired during the day.  It seems that I adjusted to Thailand time very quickly.

Near the end of my stay, I picked up a cold virus, making for a grueling 20+ hours in coach class on the way home (BTW, Korean Air serves decent food, and Incheon Airport is nicer than I expected).  Back home, it was cold and snowy, so I stayed huddled in my office all day, alone but for my computer, being exposed to almost no sunlight.  I was totally wiped out for nearly a week.

Could it be that a significant factor in jet lag is not necessarily the direction you travel, but the activities in which you participate (and the illnesses you contract, whether or not you are aware of them) at either destination?

-- JeffreyMeunier

''On the other hand, maybe it's just that you are one of the 10% for whom it doesn't matter. Research tends to trump individual personal experience.''