Note: Here's the Edited version of this entry

This is how Merriam-Webster's Medical Dictionary defines Jet Lag:

jet lag \'jet-"lag\ n : a condition that is characterized by various

psychological and physiological effects (as fatigue and irritability),

occurs following long flight through several time zones, and prob. results

from disruption of circadian¹ rhythms
in the human body -- called also

jet fatigue -- jet-lagged (adj)

Experienced travellers define it this way:

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

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

Jet lag is an abnormal condition of one's body that is encountered or rather suffered from
after Business Travels. Your body's inner clock is no more synchronized with the world around you which means that it asks for sleep when the sun shines bright in your new environment, and you find yourself staring at the ceiling of your hotel room at 3am with a stomach which demands to be fed. Otherwise the symptoms of a jet lag are close to those of a severe hangover.

The history of jet lags began with the invention of jet aircraft, of course. There being no fast transportation means before that, people crossing continents on horseback or sailing across an ocean did not even realise a time shift.

Jet lags are encountered after extended flights in east-west or west-east direction. Going north to south or vice versa is no matter of discussion because time zones have been set up perpendicular to the sun's virtual path along the equator.

Reasonable Suggestions for Avoidance

• 
  Start synchronising your inner clock some days before the flight which means to gradually delay (or advance, respectively) your daily schedule while still at home. Make sure to get lots of sleep.
  

• 
  While on the flight, walk along the aisle to keep your blood circulation going, drink lots of non-alcoholic beverages to counter dehydration in the dry air inside the aircraft. Transcontinental flights go at flight levels of some 10,000m where the atmosphere is thin and extremely cold (some -30 C or so), and therefore cannot hold significant amounts of humidity. Heating this cold air comes at a cost, therefore the air within aircraft is being recirculated: some 60 to 70 percent of the air that you breathe have been breathed out (or emitted in some other way) by a fellow passenger, only 30 to 40 percent are really fresh air. Hence the concentration of oxygen is lower than on the ground, and CO₂ levels are higher. Opening all air vents may or may not help, depending on where the fresh air is mixed into the recirculated air.
  

  
  In order to get their share of oxygen, smokers should refrain from smoking some hours before the flight, and especially on intermediate stops. Smoking on the flight is forbidden anyway, not only for reasons of air quality.
  

  
  There is also a concern about higher concentrations of viruses and other germs which transmit diseases, but airlines are reluctant to confirm this.
  

• 
  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. (Quoted from the very instructive Collection of tips by DoctorTravel).
  

• 
  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. (Another hint from DoctorTravel).
  

• 
  Stay clear of alcohol. Thin atmosphere and dry air will increase its effect by a factor of 2 or more.
  

• 
  Melatonin² tablets. Melatonin is a hormone produced by the human body during darkness and broken down at daylight. Its function is to synchronise the body's activities with the 24hour rhythm of the day, ie: it makes you sleepy in the dark and lets you awake in the sunshine. A proper dose supplied at the right time is supposed to "reset" the clock in your body.
  

  
  But instructions on a packet do not go into much detail as to which side of a tablet should point up when swallowing it on an eastbound flight. It is cheaper to make your body assume that it is midnight by closing your eyes and sleeping.
  

  
  Besides, the benefits of melatonin are controversial, according to a report [this used to be an external link. Sorry].
  

  A drug called Melatonin has been used effectively on sheep, but how many sheep do you know who are frequent flyers? - Erma Bombeck

Other Means on Offer for Recovery and Relief

• 
  Watches that speed up on eastbound flights or slow down on westbound flights, and resume their regular pace after reaching your destination.
  

  Comment: Retailers promise that you will be fully psychologically adapted to the new time zone after arrival and hope that by then you have forgotten about the price tag.
  

• 
  Self-hypnosis tapes (ie, tapes offered at hypnotising prizes) which make your mind believe there never was a warp hole in your personal space-time continuum.
  

  Comment: However, your body won't believe it.
  

• 
  Powerful lights shone on rather exotic parts of the body like the back of your knees.
  

  Comment: Maybe quite effective in curing your knees from jet lag.
  

• 
  Slide rules which are set up according to the rules of ancient chinese accupuncture. These are paperback devices which let you adjust two slides or disks according to the local times at your destination and starting point (with due consideration of daylight savings time), and devise points on your body which you ought to give a massage treatment once every hour.
  

  Comment: These devices may be useful to get acquainted to your neighbour in the row, but otherwise only keep you from sleeping.
  

Apart from the aforementioned, modern medicine has some more proposals ready for you, from Ipecacuanha to Lycopodium. Some recipes are not unlike those which are employed in wart curing, and in the middle ages they surely would have led to an indictment for witchcraft.

Conclusions

Modern science and quackery have not yet come up with a working cure against jet lag.

Therefore it is considered best to order lots of drinks on the flight, especially pernod after beer, to make good use of the remaining night hours at one's new place, and to use proven measures against hangovers afterwards.