Medical Matters:While long-haul flights were once the preserve of the globe-trotting businessman, many leisure travellers now choose to spend their holidays in far-flung destinations. Both groups face the challenge of jet lag when travelling across three or more time zones, writes Muiris Houston.
The effects of jet lag vary considerably with the individual. A close friend who lives in San Francisco appears to be largely unaffected by the phenomenon. Whether he arrives direct from the west coast of America or jets in from Singapore or Hong Kong, he is able to function quite well within hours of his arrival.
I'm not so fortunate. Even the supposedly more favourable westward direction of travel leaves me tired and unable to sleep properly.
Sleep is affected in different ways. For those travelling east, it can be difficult to get to sleep during the "new" night-time. Westward flights cause early wakening, while flights in either direction contribute to fractionated sleep (an inability to sleep continuously throughout the night).
Jet lag also reduces our ability to concentrate. And our tummies are prone to disturbances such as indigestion, diarrhoea or constipation.
Unlike travel fatigue, which settles after a good night's sleep and rehydration, jet lag takes a number of days to get over. A rough rule of thumb is that adjustment will take the number of days equal to about two-thirds of the number of time zones that have been crossed.
The severity of jet lag increases with the number of time zones crossed. It is worse for older travellers and flights to the east are associated with more severe jet lag than flights west.
Jet lag is caused by our body clock continuing to function on the day-night rhythm of the place we have left. The rhythm adapts gradually under the influence of light and dark; darkness switches on the secretion of the hormone melatonin by the pineal gland in the brain while exposure to strong light switches it off.
The body clock is located deep in the brain at either side of the hypothalamus. Nerve cells here have melatonin receptors and receive information about light from the eyes.
Sometimes called the circadian clock, it synchronises our internal systems so they function smoothly. The term circadian comes from circa (about) and diem (day) and reflects the fact that each full period or cycle is not exactly equal to 24 hours. (Latest research indicates we have a circadian rhythm of about 24.5 hours.) The clock alters hormone levels, digestion, body temperature, sleep and mood and is influenced by rhythmic cues from the outside world.
The cues are called zeitgebers (time givers) and they can produce both advances and delays in the body clock. The main zeitgebers are the light-dark cycle and the secretion of melatonin, although exercise has a weaker effect.
When we cross a number of time zones, the body clock gets confused. It has to adjust to a new time and different patterns of light and activity. But not all body functions adjust at the same rate: your sleep-wake cycle may adjust more quickly than your temperature, while your digestive system may be on an entirely different schedule.
The body clock adjusts more easily when we fly in an east to west direction. It is more natural for the internal clock to extend the day but it finds it difficult to respond to the shorter day associated with west to east travel.
How can we reduce the impact of jet lag? The key is to adjust the body clock by manipulating our light exposure and sleep.
After westward flights it is best to avoid light during the night. For example, if you have travelled five time zones to the west, then avoid exposure to light between midnight and 6am and maximise your light exposure between 4pm and 10pm. However, if you have gone through eight time zones to the east, then avoid light exposure between 5am and 11am.
A lot has been written about the benefits of melatonin in minimising jet lag. But first some warnings: it is not a licensed drug and is not available in the Republic; there is evidence that melatonin bought over the counter abroad or through the internet contains unidentified impurities. In addition, the adverse effects of melatonin have yet to be adequately investigated, but may include the risk of seizures and an interaction with anticoagulant medications.
However, the efficacy of melatonin in combating jet lag has been well researched. Taken at a dose of 3-5mg about two to three hours before bedtime, it helps induce sleep and lowers the body's core temperature, thus persuading the body to adjust its internal clock.
The effects of jet lag vary widely from person to person and individual frequent flyers have particular customs and remedies they find effective.
If readers have favourite "cures" they would like to share, I would be pleased to hear from them at mhouston@irish-times.ie
I am grateful to Dr Francis Nelson, a GP continuing medical education (CME) tutor for Co Donegal for background material used in this column.
Dr Muiris Houston is pleased to hear from readers at mhouston@irish-times.ie but regrets he is unable to reply to individual medical queries.