In search of the perfect day? Try this: work early in the day, and tackle your trickiest conundrums around noon. Do yoga or pilates at around 5pm, followed by a workout or swim. Have a glass of wine or two with an early dinner at around 7pm, and give The Ten O'Clock News a miss: instead have a long, relaxing massage, ideally followed by lovemaking.

Why? Well, according to experts, our body's systems are governed by circadian rhythms that repeat in 24-hour cycles - which means we can time our behaviour to ensure our body is best primed to deal with what we're asking of it. So morning is best for work because our short-term memory, logical reasoning and concentration peak then: yoga is good in the afternoon, because that's when our bodies are at their most flexible. In the late afternoon, body temperature peaks - some experts say this is a "natural warm-up" for exercise. Your digestive system and your liver are at their best around 7pm, and skin sensitivity and libido are highest at around 9.30pm.

Researchers are finding out more all the time about the ebb and flow of the human body: last week a new study from Long Island Jewish Medical Centre found that lung function peaks in the late afternoon: another reason why it's good to exercise then. Nearly 5,000 patients were analysed, and the findings presented to the American College of Chest Physicians were that lung function is at its least effective around midday, and at its best between 4pm and 5pm.

This has implications for more than just when you go to the gym: what lung specialists are excited by is the possibility of timing medication so that it is given at a time when it can penetrate the respiratory tissues as effectively as possible. Asthma medications, for example, could be more effective in the late afternoon: and many patients who deliver bronchodilators round the clock could find they get the same effects with a lower dose if their drugs are time-targeted.

It might also, says Long Island researcher Dr Boris Medarov, be better to extubate patients who have been on ventilators in the late afternoon, when their lung function will be at its best and they may be better able to breathe on their own.

It's the fine-tuning of drugs to the body's natural rhythms that is behind much of the current research in the field. "There are big possibilities," says science writer Leon Kreitzman, co-author (with Professor Russell Foster of Imperial College, London) of Rhythms of Life, a book about our daily rhythms. "We're learning more all the time about when is the best time to administer drugs. In the field of cancer, for example, where the drugs are very powerful, it's especially significant. What you want to do with cancer drugs is hit the tumour cells when they are dividing - the problem is, you're also hitting other areas of the body with dividing cells, such as the hair and the gut. That's why people having chemotherapy so often lose their hair and feel sick. If we could find a way of timing chemotherapy more effectively, we might be able to give the drugs at a time when the tumour cells are dividing, but when the hair cells and the gut cells are not, and so reduce the impact on those areas."

Not all the reasons why certain systems work best at certain times are fully understood, but in general, says Foster, what the research tends to show is that the human body's rhythms are linked to two external factors: light and temperature. "We live in a world that rotates once in 24 hours, and over that period there are huge changes in light and temperature," he says. "If you look at even the most primitive forms of life on the planet, you see they have a molecular clock - every aspect of life is finely tuned to cope with varying demands of our constantly changing world.

"What the body has evolved is a system of not just reacting to the changes in light and temperature, but getting ready for that change." Which is why you find that body temperature cools in readiness for sleep a couple of hours before you go to sleep (electric blankets and hot water bottles, some researchers argue, could actually inhibit the body's natural slumber-prompt), and the heart and blood systems crank up for dawn in the early hours. But even with that slow start, with the body adjusting slowly to the demands of the day, the first few hours of being up and about, with our systems still adapting to the differences between night and day, are still the most dangerous: most heart attacks, for example, happen in the first couple of hours after getting up.

The clock that controls all body systems is, says Foster, embedded within our genes - 12 or 14 genes have been identified that help regulate it with the production of certain proteins. As a result, he and Kreitzman argue, you can't simply override it: you can't expect the night-shift worker to be as productive as the day-shift worker, because that's not when we are primed to be at our best.

And although we can adapt to changes in time zones and overcome jet-lag by slowly readjusting the clock, it will still be set so that the body processes chime in with light and temperature in the part of the world we're now in.

In fact, says Professor Greg Atkinson, a sports scientist at Liverpool John Moores University, the body's natural tendency is to run on a slightly slower than 24-hour system: without any external stimuli, he says, we would tend to do things half an hour later each day.

"There have been studies done where people have been isolated from external signals such as changes in light or temperature, and what they have found is that our rhythms do persist, but they are not as strong as they are in the ordinary everyday environment," he says. "The natural body clock would probably function on a 24.5-hour cycle, rather than a 24-hour one." What that gives us, says Atkinson, is the flexibility to respond to changes such as crossed time zones, and even the minor adjustment of the clock for British Summer Time last weekend.

Atkinson's work has looked specifically at the impact of the body clock on sports performance. What he's found is that even apparently minor changes can have a big impact, such as fluctuations in body temperature. "The variation is only around 0.8 to 1 degree centigrade," he says. "It may not sound a lot, but it has a big effect - it acts like a natural pre-competitive warm-up, so your body's metabolism is faster and the rate of connection of your nerve signals is faster."

It should not surprise us, then, he says, that studies have found that people left to make up their own minds on when they undertake certain tasks tend to exercise in the early evening. Not surprising, either, that many Olympic records are made and broken in events that take place in the early evening: for athletes looking for glory, it seems, checking the schedule could be almost as important as the last-minute training.

All in the timing: Your guide to when your body works best

1-2am: Urine production is at its lowest

3-4am: Body at its lowest ebb - most likely to die

5-6am: Growth hormones peak

7-8am: Migraines most common; strokes most likely; sperm count highest - most likely to conceive

9-10am: Peak time for heart attacks; short-term memory and logical reasoning at their best

11am-12 noon: Concentration at its best

1-2pm: Peak urine production

3-4pm: Lung function at its best

5-6pm: Body at its fittest - best time to exercise; alertness peaks; body at its most flexible, muscle tone peaks

7-8pm: Liver function best; toothache peaks; digestive system at its best

9-10pm: Menopausal flushes most common; skin sensivity peaks; libido at its highest

11pm-12 midnight: Body at its most allergic; ovulation most likely