Jet lag

# Overview
Jet lag, also jetlag or jet-lag, is a physiological condition which is a consequence of alterations to the circadian rhythm. Such disturbances may result from shift work, daylight saving time, altered day length, or, as the name implies, transmeridian travel (as on a jet plane). They are known as desynchronosis, dysrhythmia, dyschrony, jet lag, or jet syndrome.  The condition is generally believed to be the result of disruption of the "light/dark" cycle that entrains the body's circadian rhythm. It can be exacerbated by environmental factors.
The condition of jet lag may last many days, and medical experts have deemed that a recovery rate of "one day per time zone" is a fair guideline. Good sleep hygiene promotes rapid recovery from jet lag: in fact sleep, relaxation, moderate exercise, and sensible diet seem to be the simplest recovery methods.
However, since the experience of jet lag varies among individuals, it is difficult to assess the efficacy of any single remedy. Age may also be a factor on the severity of jet lag, as younger people will suffer worse jet lag than older people would.
Women are more susceptible to jet lag than are men, in part because estrogen is often vulnerable to jet lag-like conditions.
Most chemical and herbal remedies, including the hormone melatonin, have not been tested nor approved by official agencies such as the U.S. Food and Drug Administration.
A recent study in hamsters showed that sildenafil (also known commercially as Viagra) aided in a 50% faster recovery from shifts comparable to eastward travel experienced by humans and was effective starting at low doses.  However, this use has not been tested in humans and is considered an off-label use by the drug's manufacturers.

# Causes
When traveling across a number of time zones, the body clock will be out of sync 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 no longer correspond to the environment nor to each other in some cases.  "Jet lag" describes the physical symptoms that occur because the body cannot immediately realign these rhythms.  The speed at which the body readjusts itself to new daylight/darkness hours varies among individuals. Thus, while it may take several days for some people to readjust to a new time zone, others seem to experience little disruption to their body's natural patterns. It is, however, not common to get jet lag from a crossing of only one or two time zones.
There have been links made with jet lag and contamination of cabin air by TCP or Tricresylphosphate from contaminated engine bleed air, which supplies the cabin pressurisation system, although these have not been substantiated, and aircraft manufacturers are adament that air quality is generally high.

# Symptoms
The symptoms of jet lag can be quite varied, though on the whole, an individual may experience the following:
- Dehydration (possibly due to the dryness of air on planes)
- Loss of appetite
- Nausea and/or upset stomach
- Headaches and/or sinus irritation
- Fatigue
- Disorientation and/or grogginess
- Insomnia and/or highly irregular sleep patterns
- Irritability, irrationality
- Mild depression
Frequent changes of time zone, working long hours, or rotating shifts causes a loss of productivity, alertness, and safety costing an estimated $70 billion per year.
The condition is not linked to the length of flight, but to the transmeridian (i.e., east-west) distance traveled. Hence, a ten-hour flight between Frankfurt and Johannesburg (going south, staying roughly on the same meridian) is much less inducive of jet lag than a five-hour flight between New York and Los Angeles or vice versa. Also, the International Date Line should not be confused as contributing to jet lag, as the maximum possible disruption is plus or minus 12 hours.  If the time difference between two locations is greater than 12 hours, subtract that number from 24. (For example, a 20 hour time difference equals 4 hours of jet lag.) Layovers can complicate this simple arithmetic, however.
Jet lag can be especially difficult near the north and south poles, where there are extreme periods of daylight or darkness, depending on the time of year, which often causes insomnia or hypersomnia.

# Direction of Travel
There seems to be some evidence that traveling west to east is the more disruptive.  This may be because flights to the east run counter to the circadian rhythm. Most people have a circadian period which is a bit longer than 24 hours, making it easier to stay up later than to get up earlier.
It may also be that flights to the east are more likely to require people to stay awake more than one full night in order to adjust to the local time zone. For example, comparing a typical schedule for a traveler flying to the East vs a traveler flying to the West:
- Westward from London to Los Angeles, VIA BA0279, Jan 29, 2008. Time zone difference 8 hours.
- Eastward from Los Angeles to London, VIA BA0278, Jan 29, 2008.
The first scenario is equivalent to staying up all night and going to bed at 6am the next day8 hours later than usual. But the second scenario (eastward) is equivalent to staying up all night and going to bed at 2pm the next day16 hours after the time one would otherwise have gone to bed.

# Net Lag
People depending of updated information from other time zones, may suffer from biological clock variations causing them to be less time adjusted to the social routines of the surrounding geographical region and may suffer from similar symptoms of jet lag during the adaptation period.