International travel, shift work, late nights out - they all mess with your body clock, and your health can suffer. Tobias Jones investigates
Deep in your brain, lodged just behind your eyes, there is a literal body clock. All humans have one: the clock is called a supra-chiasmatic nucleus, and scientists can isolate it, plug it up to electrodes, and convert the ticking into a regular, electronic beep. The clock can govern anything subject to our 24-hour routine: body temperature, hormone cycle, patterns of alertness and tiredness. Because it influences the day-by- day, it has a knock-on effect for the longer term, probably prompting the body's menstruation and reproductive cycles and helping our adjustment to seasonal changes.

The trouble, scientists are now warning, is that this sensitive piece of biological timekeeping is being skewed by humans' unnatural lifestyles. International travel across different time zones, more and more shift work (20 per cent of Londoners work shifts) and late-night leisure pursuits take no heed of that little nucleus. Increasingly the body clock is being masked (by, for example, caffeine or amphetamines) or simply ignored.

"I think we should be very concerned about our 24-hour society," says Jo Arendt, a professor of endocrinology at the University of Surrey. Her conclusion is that if you mess with your natural tick tock you could hurt your heart. "There's a higher incidence of cardiovascular disease in shift workers: they have higher fat in blood samples, because the body can't handle the same meal at odd times. There's evidence of increased gastro- intestinal diseases, ulcers, and of course more likelihood of accidents and social and family problems."

Given that we are bound to continue living our arrhythmic lives, scientists are now concentrating on ways to reset our clocks, to accommodate them to our irregular hours. Inevitably, the American military have become interested, and are funding a research project under Dr Philis Zee of North Western University, investigating what she believes to be a family of human genetic mutants, living on a 20-hour cycle body clock.

Because the minutiae of the body clock, the intricacies of its timekeeping, are mostly inherited and therefore part of your genetic make up, Philis Zee is hoping to isolate the particular strand of DNA responsible for the clock. Already a mouse with a genetic clock mutation has been found and it was possible to locate its rogue gene. The discovery of the human equivalent would have huge implications: from birth you could understand your ideal circadian rhythm. In the same way that cars go at different speeds, humans have variant cycles. It could be known whether you are on a "long" or "short" cycle, whether you are a "lark" or an "owl". This would enable people to know at what times of day they might be most alert, and when most drowsy. You would be able to predict at which hour you would most profitably take a driving test, and what time to avoid interviews. It might even enable clockwork menstruation and perfect sleep patterns.

Step forward the Blumberg family, the guinea pigs in the laboratory. Three generations of this family from America claim to be genetic mutants: they believe that they live on a 20-hour clock. They are in bed by 8pm almost every evening, and wide awake by 3am. They only ever attend matinees in the theatre, and can meet friends only for lunch or very early supper.

Carol Sweig's father kept these unusual hours, as have she and her offspring. Her daughter, Heidi, worries about her ability to stay up late when her children reach their teenage years: "Midnight is literally the middle of the night. I cannot be awake at that hour; to me, it feels abnormal."

Until Zee finds the elusive gene, if you do lead an erratic lifestyle, how do scientists suggest you can best adapt your body? The main "cue" for the clock, the factor which adjusts it to 24 hours - its natural cycle is actually 24hrs 20 mins - is light. (Our "clock" is particularly close to the retina, which is why blind people, their natural timekeeping unprompted by daylight, have a much higher incidence of sleeping disorders.) So the best way to adapt your body to unusual hours is by providing it with an intensity of light which might otherwise be absent. Nasa uses strong light to "shift" its astronauts, and many night-time workplaces in the States have "statutory and strong illumination".

Another, closely related way of adapting the clock is by using melatonin. It is a naturally occurring "chrono-biotic"; a hormone-like secretion from the pineal gland, melatonin's basic function is to tell your body "when the dark happens". It also reduces body temperature, giving it its soporific quality. "It's a darkness hormone," says Professor Arendt, Britain's leading expert on the subject, and someone who has been taking and researching melatonin since the early 80s. "It is important," she says, "for all animals who use the length of night to determine reproduction or sleep cycles." Available on prescription, melatonin is widely used by those suffering jet-lag, or shift workers needing to sleep in the day time.

`What Make Us Tick' , a programme about the body clock, is broadcast next Saturday at 6.50pm on BBC2.