Jet lag, or desynchronosis, is a common consequence of air travel. It is a disruption of the body’s internal clock that can have a significant impact on your psychological and physical well-being, but only for a few days. Jet lag occurs only when you travel across time zones and there are steps you can take to lessen its effects on your body clock.

Jet lag and circadian rhythms

The body’s internal clock is governed by circadian rhythms. These control hormone levels and affect sleepiness, wakefulness, hunger and digestion. Your circadian rhythms are determined by habit, but only to an extent. Exposure to sunlight also matters: sunlight fosters alertness while darkness fosters sleep. Crossing time zones alters the amount of sunlight you receive and disrupts your circadian rhythms, so leads to jet lag.

The effects of jet lag

Jet lag affects different people in different ways but common symptoms include:

  • Psychological symptoms: fatigue and tiredness, mild headaches, mild depression, anxiety, irritability, poor concentration, confusion, memory loss, clumsiness, lack of energy and insomnia.
  • Digestive problems: constipation, diarrhoea, nausea and loss of appetite.
  • Menstrual problems: frequent flying can disrupt the menstrual cycle.

These symptoms start after a quick transition between time zones, which typically occurs when you fly from one side of the Atlantic to the other. Long plane journeys in themselves can cause muscle cramps, itchy eyes, a runny nose, swollen feet and tiredness. These are caused in part by the low level of air pressure within jet planes, and in part by staying seated for long periods. Although such problems are not in themselves jet lag, they may contribute to the overall symptoms.

As a rough guide, jet lag lasts about a day for each time zone crossed up to about five days, after which time most jet lag disappears no matter how long the journey. Symptoms tend to last longer when you travel west to east.

When is jet lag worse?

No two journeys are the same but some factors make jet lag much more intense:

  • Number of time zones crossed: journeys that cross fewer than three time zones are unlikely to cause jet lag. Beyond this, the more time zones you cross, the worse your jet lag will be.
  • Axis of travel: time zones do not change north–south; they only change east–west. So you’ll get jet lag only if you travel east–west. For example, Sweden and South Africa are in the same time zone; Sweden and California are nine time zones apart. Flying from Sweden to South Africa does not cause jet lag, but flying from Sweden to California does.
  • Direction of travel: travelling east to west causes more jet lag than travelling west to east for two reasons. Firstly, your natural circadian rhythm is slightly longer than 24 hours, and when you travel east to west your day becomes longer, so you are travelling in the direction your body ‘wants to go’; the converse is true when you travel west to east. Secondly, because travelling east to west lengthens your day, you are likely to be exposed to more sunlight. Such differences have real effects. Athletes who travel west to east perform less well than those who travel east to west.

Avoiding jet lag

You can minimise jet lag using several strategies:

  • Before travel, gradually adjust to the time zone of your destination: if you are travelling east to west, go to bed progressively later than usual because travelling east to west lengthens your day. Conversely, if you are travelling west to east, go to bed progressively earlier than usual. Similarly, adjust your meal times to accord with those of your destination. Ideally, you should start adjustment three to four weeks before your journey. If you use regular medication, consult with your medical doctor on how to reschedule it.
  • Before travel, make sure you are in good shape: your body copes best with jet lag when it is strong. So help your body by eating healthy foods, taking physical exercise, and avoiding unhealthy activities such as binge drinking. Drinking alcohol on the actual journey does not help at all.
  • During travel, help your body cope with the flight: avoid dehydration by drinking water rather than alcohol or caffeine; shift position and get up from time to time in order to stretch your legs; break up your journey, if possible, by an overnight stay in a midway destination; catnaps when travelling east to west may also help.
  • Adjust to your local time zone when you arrive: adjust to normal times of eating and sleeping straight away but don’t overeat and try to avoid alcohol, particularly if you feel jet lagged. It can also help to expose yourself to sunlight at specific times of the day depending on your direction of travel and the number of time zones crossed.
  • Travelling west–east: for flights of fewer than six times zones crossed, get sunlight in the morning, but for more than six time zones crossed, wait until noon.
  • Travelling east–west: for flights of fewer than six time zones crossed, get sunlight in the late afternoon, but for more than six time zones crossed, get sunlight at noon but avoid it in the afternoon.

There are claims that taking melatonin, a hormone implicated in the sleep–wake cycle, relieves jet lag. Evidence that it does so is at best mixed and it does seems unlikely that this will become an accepted jet lag treatment.