Jet lag (or "jet-lag") is a physical condition caused by crossing multiple time zones during flight. The condition is generally believed to be the result of disruption to the circadian rhythms (i.e. the "light/dark" cycle) of the body. It can also be exacerbated by experiencing sudden changes in climate or seasonal conditions, as well as the reduced oxygen, partial pressure and low humidity commonly experienced in the cabin of an aircraft.

Characteristic symptoms of jet lag include:

• Fatigue
• Disorientation and/or grogginess
• Nausea and/or upset stomach
• Headaches and/or sinus irritation
• Insomnia and/or highly irregular sleeping patterns
• Dehydration and loss of appetite
• Irritability
• Irrationality

The condition is not linked to the length of flight, but to the east-west distance travelled. Hence a ten hour flight between Oslo and Johannesburg will probably be less inducive of jet lag than a four hour flight between New York and Los Angeles. There is no firm agreement as to which direction of travel is worse. Some believe that travelling east is worse as it "accelerates" the passage through various time zones (a night might only last 3 hours when flying east). Others counter that the impact is worse when one travels away from one's "habitual" time zone, and is minimised when returning to it.

Prevention and recovery

Recent research shows that a pineal hormone, melatonin, is potentially effective in prevention and treatment of jet-lag. It appears to be safe in short term use. [1] (http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12076414&dopt=Abstract) Many remedies of various nature are marketed but rarely effective. Their safety has not been scientifically established. For people who don't fly multiple times per week, it can be an effective non-drug remedy to skip sleep entirely for one night and one day and then go to bed at a normal destination-area bedtime on the next day.

The condition of jet lag generally lasts a few days or more, and medical experts have deemed that a recovery rate of "one day per time zone" is a fair guideline. Sleep, relaxation, moderate exercise and sensible diet seem to be the simplest recovery agents.

Good sleep hygiene promotes rapid recovery from jetlag.