Our body clock helps us anticipate and respond to nature's rhythms, particularly the 24-hour cycle of day and night.

It ensures that the body's organs and systems work in harmony with one another and in sync with the external world.

Modern life
But modern life does not run according to nature's rhythms. We no longer wake up with the sunrise and go to bed when it gets dark. Our lives are governed by artificial clocks that tell us when to get up, when to eat, when to start and finish work, and when to sleep.

In today's 24/7 society, we use alarm clocks to jolt us from our sleep, electric lighting to override the natural cycle of day and night, and endless cups of coffee to keep us artificially alert.

Normally our circadian clocks adapt well to the demands of modern life and keep our physiological processes ticking along nicely. But sometimes - for example, when we fly long distances or have to do shift work - our biological clocks can struggle to cope.

Jet lag
The body clock operates on a period of slightly more than 24 hours. Left to itself it would soon fall out of sync with the external world. But external cues, principally the coming of daylight and the onset of darkness, are used to reset our circadian clocks and keep us synchronised with day and night.

This system is quite flexible. It enables us to cope with seasonal changes in day-length (not so important in Singapore, which lies near the equator, but vital for those living in northerly or southerly latitudes). It also helps us adjust to the demands of modern living, since electric light resets our circadian clocks in the same way that daylight does.

Despite this flexibility, our circadian clocks cannot adjust themselves quickly to major disruptions in the pattern of day and night, such as those we experience during a long-haul flight.

For example, a flight from London to Singapore leaves our circadian clock lagging seven hours behind external time. This means that when the clock on the wall says 10am, the body clock reads 3am.

This is a major shift, and it takes the body clock several days to adjust to the new rhythm. During the transition, travellers often suffer what is called jet lag. Symptoms include insomnia, fatigue, headaches, irritability and difficulty concentrating.

Anyone who travels regularly will know that jet lag is worse when travelling west to east than it is when travelling east to west. The body clock finds it easier to cope with an extended day than a reduced one.

One of the most effective ways of reducing jet lag is to seek or avoid light at certain, carefully calculated times. Light is the primary cue that links the circadian clock with the external world. So carefully controlled exposure to light can help smooth the transition to the new time zone. For more information, see the 'jet lag calculator' at www.britishairways.com/travel/drsleep/public/en_gb

Shift work
Although jet lag can be very unpleasant, its effects are short-lived. Within days, the circadian clock adjusts to the new time zone and full health is restored.

People who work rotating shifts have it much worse. Their circadian rhythms are constantly disrupted by changing patterns of light and darkness, and work and sleep. As a result, many of them suffer 'shift lag'. Unlike jet lag, this is a long-term condition and therefore much more injurious to health.

Shift workers get less sleep than the rest of us. They are more likely to be obese; more likely to suffer cardiovascular and gastrointestinal problems; more likely to be involved in workplace and motor vehicle accidents; more likely to suffer from work-related stress; more likely to suffer from depression; and more likely to suffer family problems, including divorce.

Clearly we pay a price for overriding our body's natural rhythms.

The most obvious solution to the problem is for workers to try to avoid rotating shifts. But this is often impractical. So workers and employers must find ways to minimise shift work's harmful effects.

For example, it is usually recommended that shifts rotate forward from day to afternoon to night, rather than backward from day to night to afternoon, since the body clock copes better with moving ahead than moving back.

Detailed, practical advice is available at the Canadian Centre for Occupational Health and Safety's website at www.ccohs.ca/oshanswers/work_schedules/shiftwrk.html

E-mail: gary@garyhayden.co.uk

Gary Hayden is a freelance writer whose specialisations are: education, science & philosphy, health & wellbeing, travel and short fiction.

This article was first published in the Mind Your Body supplement on Jan 16, 2008.