What causes jet lag?

There must be more to it then just time difference, as I do shift work and am used to changes in my sleep patterns, so I seem to be more exhausted then usual after a 7 hour flight

When traveling across a number of time zones, the body clock goes out of sync with the destination time, and so it experiences daylight and darkness contrary to the rhythms to which it has grown accustomed. It is not common to get jet lag from a crossing of only one or two time zones. In jet lag, the body's natural pattern is upset, as the rhythms that dictate times for eating and sleeping no longer correspond to the environment of the destination. Jet lag occurs because the body cannot immediately realign these rhythms. The speed at which the body readjusts itself to new daylight/darkness hours and eating/sleeping patterns is entirely dependent upon the individual. Thus, while it may take a few days for some people to readjust to a new time zone, others seem to experience little disruption to their body's natural sleeping patterns.

Becci is right, it is the difference crossing different time zones. I travel twice a month 10 - 12 hr flight then go into working night duty for three weeks. It takes me approx 48hrs to recover properly, my body us used to the travel and working night shift but it does not get any easier the older you are!.