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Sleep and dreams
The Sleep/ Dream/ Wake Cycle
Our Biological Clocks

Help Link : Jet Lag Link : Jet Lag Link : Voyage et décalage horaire
Link : Phase-dependent effect of room light exposure in a 5-h advance of the sleep-wake cycle: implications for jet lag Link : Beating Jet Lag

The maximum jet lag that you can experience is 12 hours. If the difference between your flight’s departure point and its destination exceeds 12 time zones, then you have to subtract the actual number of time zones from 24 to calculate the actual number of hours of jet lag that you will feel. For example, if you are flying from Los Angeles to Hong Kong, you will pass through 16 time zones, so you will experience 24 - 16 = 8 hours of jet lag—the same as if you had flown through 8 time zones from, say, London, England to Los Angeles.

For flights that cross the International Date Line, the calculation is the same, but you do not have to take the date change into account.


Industrial civilization, with innovations such as night shiftwork, variable shiftwork, and intercontinental air travel, has subjected the human brain to conditions that evolution never foresaw.

For instance, when a businesswoman arrives in New York on a flight from Paris, her endogenous circadian rhythms are still synchronized with the time of day in the French capital, but the ambient light signals that her brain is receiving match the time of day in the Big Apple, which is 6 hours earlier. Thus her need for sleep is out of phase with the local time at her destination.

The uncomfortable symptoms known as jet lag generally start to appear when the departure and arrival points for a flight are three or more time zones apart. The most obvious of these symptoms is sleepiness in the daytime and wakefulness in the middle of the night. But other symptoms may include fatigue, loss of appetite, indigestion, headaches, nausea, irritability, and a bad mood.

Over 75% of air travellers whose flights cross several time zones report problems in sleeping the first night afterward. But after three nights, this figure is down to 30%. It takes just about one day to recover for every time zone that you travel through, but this figure varies widely from one person to the next. People who have a very regular routine when they are at home are more sensitive to jet lag than people who lead less structured lives.

People do not experience jet lag when they fly from north to south or south to north, even on long-haul flights such as from Lima, Peru to New York (flight A in the diagram below). Because these people are staying in the same time zone, their biological clocks are not thrown off, and all they have to deal with is the fatigue due to the long period of immobility, which is rapidly overcome.

On this map, flight B, from New York to Paris, passes through 6 time zones and will cause its passengers
moderate jet lag that takes them two to three days to overcome. Flight C, from Copenhagen to Alaska,
passes through 11 time zones, and even though it is westbound rather than eastbound (see below), will cause
very heavy jet lag that will take a good week to dissipate.

Link : Les vols transméridiens : Paris - New York

Westbound long-haul flights are often said to cause less jet lag than eastbound ones passing through the same number of time zones. Many explanations have been offered to support this assertion. One, which applies only to eastbound flights departing in the evening, is that passengers on such flights may get only a few hours of darkness in which to sleep before seeing the next sunrise.

Another explanation is that because the human biological clock’s endogenous cycle is naturally slightly more than 24 hours long, and because westbound flights through more than one time zone lengthen the passengers’ days, such flights are easier for the body to recover from than eastbound ones. But this hypothesis would seem to contradicted by experiments in which westbound and eastbound time lags were simulated for animals whose endogenous cycle was almost exactly 24 hours, and these animals too experienced fewer physiological disturbances in the westbound case.

In short, when it comes to jet lag, so many variables seem to be involved and there seems to be so much variability among individuals that it is impossible to state categorically which direction of travel is more difficult.

The usual advice for reducing the effects of jet lag is as follows.


Link : L'horloge biologique et l'effet «jet-lag»

  • Start to adjust your body to your destination time zone in advance. If you will be flying east, go to bed earlier and get up earlier. If you will be flying west, go to bed later and get up later. The extra time you spend awake in the morning in one case and in the evening in the other will be more effective if you spend it in a brightly lit environment.
  • As soon as your flight takes off, set your watch to the time at your destination.
  • Once you arrive at your destination, expose yourself to sunlight to help your biological clock reset itself more quickly.
  • During the days following your arrival, be physically active and eat a balanced diet.
  • And perhaps most important, make sure you are well rested before your flight.

This advice is based on today’s scientific understanding that jet lag consists of a desynchronization between our central biological clock and our multiple peripheral biological clocks.

There is much controversy about whether melatonin can be an effective treatment for the symptoms of jet lag. In some studies, people have reported positive effects when they took a dose of 0.5 mg to 5 mg of melatonin at bedtime, local time, for the first few days after they arrived at their destination. But other studies have found that melatonin had none of the desired effects, nor any undesirable side effects, at least in the short term.

In some countries, such as the United States, melatonin is freely sold as a food supplement and represents a gigantic market, worth $US 200 to 300 million per year. These supplements may help to regulate sleep and are therefore widely used by insomniacs.

Link : La mélatonine est efficace, mais il faut la prendre à la bonne heure ! Link : Mélatonine et décalage horaire Link : Mélatonine : pas de danger, mais pas de bénéfice non plus! Link : The Facts on Melatonin and Jet Lag Link : No Evidence That Melatonin Is Effective In Treating Jet Lag Link : Latest Research says using Melatonin for jet lag doesn't counter jet lag Link : What is melatonin and what’s it good for? Link : Mélatonine : le miracle se fait attendre
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