Causes & Symptoms
When traveling across a number of time zones, the body clock will be out of synchronisation with the destination time, as it experiences daylight and darkness contrary to the rhythms to which it has grown accustomed: the body's natural pattern is upset, as the rhythms that dictate times for eating, sleeping, hormone regulation and body temperature variations no longer correspond to the environment nor to each other in some cases. To the degree that the body cannot immediately realign these rhythms, it is jet lagged.

The speed at which the body adjusts to the new schedule depends on the individual; some people may require several days to adjust to a new time zone, while others experience little disruption. Crossing one or two time zones does not typically cause jet lag.

The condition is not linked to the length of flight, but to the transmeridian (east-west) distance traveled. A ten-hour flight from Europe to southern Africa does not cause jet lag, as travel is primarily north-south. A five hour flight from the west to the east coast of the United States may well result in jet lag.

Crossing the International Date Line does not contribute to jet lag, as the guide for calculating jet lag is the number of time zones crossed, and the maximum possible disruption is plus or minus twelve hours.

The symptoms of jet lag can be quite varied and may include the following:[2]

Loss of appetite, nausea, digestive problems
Headache, sinus irritation
Fatigue, irregular sleep patterns, insomnia
Disorientation, grogginess, irritability
Mild depression

Direction of travel
There seems to be some evidence that traveling west to east is more disruptive. This may be because most people have a circadian period which is a bit longer than 24 hours, making it easier to stay up later than to get up earlier.[3]

It may also be that flights to the east are more likely to require people to stay awake more than one full night in order to adjust to the local time zone. For example, comparing a typical schedule for a traveler flying to the East vs a traveler flying to the West:

Westward from London to Los Angeles, VIA BA0279, Jan 29, 2008. Time zone difference 8 hours.
Westward Biological clock Los Angeles local time
Departure JAN 29 - 10:05 JAN 29 - 02:05
Arrival JAN 29 - 21:10 JAN 29 - 13:10
Bedtime JAN 30 - 06:00 JAN 29 - 22:00

Eastward from Los Angeles to London, VIA BA0278, Jan 29, 2008.
Eastward Biological clock London local time
Departure JAN 29 - 15:59 JAN 29 - 23:59
Arrival JAN 30 - 02:05 JAN 30 - 10:05
Bedtime JAN 30 - 14:00 JAN 30 - 22:00

The first scenario is equivalent to staying up all night and going to bed at 6am the next day — 9 hours later than usual. But the second scenario (eastward) is equivalent to staying up all night and going to bed at 2pm the next day — 14 hours after the time one would otherwise have gone to bed.
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