Physicists from the University of Maryland (UMD) have developed a new mathematical model that attempts to answer why it is more difficult for people to fight off the effects of jet lag when flying eastward.

In a study featured in the American Institute of Physics' (AIP) journal Chaos, physics professor Michelle Girvan and her colleagues at UMD describe how certain cells in the brain known as neuronal oscillator cells tend to be affected whenever a person crosses into a different time zone.

Neuronal oscillator cells are the ones responsible for regulating the body's circadian rhythm (biological clock) by synchronizing with each other as well as with external cues.

However, as Girvan pointed out, these cells don't seem to operate perfectly on a 24-hour schedule. They follow a slightly longer work cycle instead, which typically lasts for 24.5 hours. This means that a traveler would find an easier time extending the length of his or her day by flying westward across other time zones than trying to shorten the day by flying eastward.

After incorporating the activity of neuronal oscillator cells into their mathematical model, Girvan and her team found that the popular notion of giving oneself at least a day to recover for every time zone crossing may not be as effective at treating jet lags as initially believed.

Aside from considering the number of times a person crossed into a different time zone, the researchers believe the direction in which he or she traveled to should also be taken into account to determine the adjustment period needed.

The team's analysis suggests that if an individual traveled westward and crossed three different time zones, he or she would need about four days in order to fully adjust to the time change.

The length of the recovery period would then adjust depending on the number of time zones crossed. A traveler would need six days to recover from a six-time zone crossing and less than eight days to recover from a nine-time zone crossing.

People who travel eastward, however, would need to follow a different recovery period compared to those who went westward.

Those who crossed three time zones eastward would need a little over four days in order to adjust to the time change, while those who crossed six time zones would need more than eight days to do so. Crossing nine time zones would require more than 12 days of recovery time for the traveler.

For journeys that would take travelers across 12 time zones, regardless of whether it is eastward or westward, the recovery time would be about nine days.

The researchers pointed out that not every person has a biological clock that operates at 24.5 hours. Some individuals follow a different circadian rhythm compared to others.

Girvan said that external cues should also be considered when it comes to finding out the recovery period for jet lag. One such factor is the individual's reaction to sunlight, which could affect how long it would take for him or her to adjust to the new time zone.

Girvan and her colleagues believe that their new mathematical model can be used in the future to help figure out a solution for jet lag.