Time zones in Europe:
WET Western European Time Zone +0
Great Britain, Ireland, Iceland, Portugal
CET Central European Time Zone +1
Sweden, Norway, Denmark, Poland, Germany, Czech, Slovakia, Austria, Hungary, Coratia, Slovenia, Spain, France
EET Eastern European Time Zone +2
Finland, Estonia, Latvia, Lithuania, Romania, Bulgaria
When travelling across a number of time zones, the body clock will be out of synchronization 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 trans-meridian (west–east) 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 east to the west coast of the United States may well result in jet lag. Travelling east causes more problems than travelling west because the body clock has to be advanced, which is harder than delaying it, and the necessary exposure to light to realign the body clock does not tie in with the day/night cycle at the destination. Travelling west causes fewer problems than travelling east, and it is usually sufficient to seek exposure to light during the day and avoid it at night.