Merriam-Webster's Medical Dictionary defines jet lag as:

... a condition that is characterized by various psychological and physiological effects (such as fatigue and irritability), occurs following long flights through several time zones, and probably results from disruption of circadian¹ rhythms in the human body. Called also jet fatigue.

Experienced travellers define it this way:

I hate jet lag. That's when you arrive and your luggage is in better shape than you are.
- Gene Perret²

and:

Jet lag is nature's way of making you look like your passport photo.
- Linda Perret³

Having Jet Lag

Jet lag, the bane of Business Travel, occurs when your body's inner clock is no longer synchronized with the world around you; which means that it asks for sleep when the sun is shining, and you find yourself staring at the ceiling of your hotel room at 3am with your stomach demanding to be fed. Otherwise the symptoms of jet lag are close to those of a severe hangover.

The history of jet lag began with the invention of jet aircraft, of course, there being no means of transportation fast enough to cause it before then. People crossing continents on horseback, or sailing across an ocean, did so too slowly to appreciate a time shift.

Jet lag is encountered after extended flights in an east-west or west-east direction. Going north to south (or vice versa) does not cause it, because time zones have been set up in accordance with the sun's apparent passage across the sky. Therefore, locations close to the same line of longitude will usually be in the same time zone.

Avoiding Jet Lag

• Start synchronizing your inner clock some days before the flight, which means gradually altering your daily schedule while still at home. Make sure that you get lots of sleep.

• While on the flight, walk along the aisle to keep your blood circulation going and drink lots of non-alcoholic beverages to counter dehydration in the dry air inside the aircraft.

• On international travel, seasoned passengers either book overnight flights when heading east, so they can sleep most of the flight, or flights that arrive at night, so they can go to bed at their destination.

• On the plane change your watch to your destination time zone. Sleep on the plane if it is bedtime in your arrival city. Stay awake if it isn't.

• Stay clear of alcohol. Thin atmosphere and dry air will increase its effect, and end up making you feel even worse.

Relief for the Desperate

• Melatonin is a hormone produced by the human body during darkness and broken down in daylight. Its function is to synchronize the body's activities with the 24-hour rhythm of the day; it makes you sleepy in the dark and lets you wake in the sunshine. A proper dose of Melatonin tablets, taken at the right time, is supposed to 'reset' your body's clock. Consult your doctor.

• Watches that speed up on eastbound flights or slow down on westbound flights, and resume their regular pace after reaching your destination are designed to allow the wearer to gradually adjust to the new time zone. Retailers promise that you will feel so good about this that you will have forgotten about the price tag.

• Self hypnosis tapes are intended to convince you that the time hasn't changed anyway. Your body won't believe it, however.

• Powerful lights shone on parts of the body, like the back of your knees, are said be effective... at least curing your knees of jet lag.

• Slide rules set up according to the rules of ancient Chinese acupuncture let you adjust two slides or disks according to the local times at your starting point and destination (with due consideration of daylight savings time), and select points on your body which you ought to give a massage treatment to once every hour. These devices may be a useful way to get acquainted to your neighbour in the row, but otherwise may only keep you from sleeping.

• Ipecacuanha, a Brazilian plant traditionally used to treat amoebic dysentery, and Lycopodium, a North American moss traditionally used by native healers to treat skin problems (and by photographers to make flash powder) are used in the manufacture of herbal jet lag remedies. Camomile tea is also said to be effective; it will, at least, make you more relaxed about having jet lag. Some herbal remedies are disquietingly similar to those which were employed in wart removal during the Middle Ages, when they surely would have led to accusations of witchcraft. Modern science and quackery have not yet come up with a practical, working cure against jet lag.