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Jet lag, medically referred to as "desynchronosis," is a physiological condition which is a consequence of alterations to circadian rhythms; it is classified as one of the circadian rhythm sleep disorders. Jet lag results from rapid long-distance transmeridian (east-west or west-east) travel, as on a jet plane.

The condition of jet lag may last many days, and recovery rates of 1 day per eastward time zone or 1 day per 1.5 westward time zones are mentioned as fair guidelines.^[1]

Cause

When traveling across a number of time zones, the body clock will be out of synchronisation 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 variations no longer correspond to the environment nor to each other in some cases. To the degree that the body cannot immediately realign these rhythms, it is jet lagged.

The speed at which the body adjusts to the new schedule depends on the individual; some people may require several days to adjust to a new time zone, while others experience little disruption. Crossing one or two time zones does not typically cause jet lag.

The condition is not linked to the length of flight, but to the transmeridian (east-west) distance traveled. A ten-hour flight from Europe to southern Africa does not cause jet lag, as travel is primarily north-south. A five hour flight from the west to the east coast of the United States may well result in jet lag.

Crossing the International Date Line does not contribute to jet lag, as the guide for calculating jet lag is the number of time zones crossed, and 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. Note, for example, that the time zone GMT+14 will be at the same time of day as GMT-10, though the former is one day ahead of the latter.

Symptoms

The symptoms of jet lag can be quite varied, depending on the amount of time zone alteration. They may include the following:^[2]

• Digestive problems
• Headaches
• Fatigue, irregular sleep patterns, insomnia
• Disorientation, grogginess, irritability
• Mild depression

Other symptoms which some may attribute to jet lag, such as nausea, ear aches and swollen feet, may be caused by the mode of travel rather than the time zone change.^[3]

Direction of travel

There seems to be some evidence that for most people, traveling west to east is more disruptive. This may be because 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.^[4]

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 traveller flying to the East vs a traveller flying to the West:

• Westbound from London to Los Angeles, VIA BA0279, Jan 29, 2008. Time zone difference 8 hours.

Westbound	Biological clock	Los Angeles local time
Departure	JAN 29 - 10:05	JAN 29 - 02:05
Arrival	JAN 29 - 21:10	JAN 29 - 13:10
Bedtime	JAN 30 - 06:00	JAN 29 - 22:00

• Eastbound from Los Angeles to London, VIA BA0278, Jan 29, 2008.

Eastbound	Biological clock	London local time
Departure	JAN 29 - 15:50	JAN 29 - 23:50
Arrival	JAN 30 - 02:00	JAN 30 - 10:00
Bedtime	JAN 30 - 14:00	JAN 30 - 22:00

The first scenario is equivalent to staying up all night and going to bed at 6am the next day — 8 hours later than usual. But the second scenario (eastward) is equivalent to staying up all night and going to bed at 2pm the next day — 14 hours after the time one would otherwise have gone to bed. Some sleep onboard may help the situation somewhat.

The Red-eye flight is another eastward scenario, for example flights departing the west coast of the USA at midnight (PST/PDT) and arriving on the east coast early in the morning (EST/EDT). Relative to the shorter flight time and the time zones advanced, the body gets less than optimal rest to begin a day of activity.

Remedies

Since the experience of jet lag varies among individuals, it is difficult to assess the efficacy of any single remedy. Gradual adjustment over the course of several days of the onset of sleep while maintaining its regular length of 7–8 hours can reduce fatigue and prevent depression. When the goal is to catch-up with local time (vs. fallback to), this can be aided by avoiding afternoon naps and eating an early and carbohydrates-rich, low-protein dinner.^{[citation needed]}

Most chemical and herbal remedies, including the hormone melatonin, have not been tested nor approved by official agencies such as the United States Food and Drug Administration. Few studies have tested the use of melatonin for jet lag and have given mixed results, likely because the timing of administration needs to be precise and individualized.^{[citation needed]}

A recent study in hamsters showed that sildenafil citrate (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.^[5] However, this use has not been tested in humans and is considered an off-label use by the drug's manufacturers.

The presence of low-level light at night also accelerates recovery rate in both east- and west-travelling hamsters of all ages by 50%; this is thought to be related to simulation of moonlight.^[6]

See also

• Chronobiology
• Light therapy
• Red-eye flight, an overnight east-bound flight from the west coast.

References