You want a baby? Then scrap candle-lit dinners, abandon late-night passions, ignore early-morning urges, and simply home in on 5.15pm, the time of day that doctors now believe is the optimum for making babies.

Yes, researchers have discovered that between 5pm and 5.30pm the average man is 35% more potent than he is at 7am. He may have as many as 17 million more highly mobile spermatozoa swimming around in the late afternoon than in the early morning. An added advantage is that the period between 3pm and 7pm is also known to be the peak for ovulation.

In the first research of its kind, a team of doctors from the University of Modena in Italy asked male volunteers with an average age of 31 to keep daily diaries and abstain from sex for three days before having the quality of their sperm tested over a 24-hour period. Reporting their findings in the medical journal, Human Reproduction, the team say that the afternoon peak in male potency makes it the best time for 'spontaneous conception', and they urge anyone with infertility problems to give the strategy a try.

Although there are known to be seasonal variations, this is, the researchers point out, the first time that daily changes in semen quality have been recorded. But such peaks and troughs should not really come as a surprise because the overall performance of the human body varies enormously over a 24-hour period.

Like the many other ups and downs in the body's circadian cycle, the variations in male potency are governed by constantly changing outputs of hormones, nutrients, amino acids, and myriad other biological compounds whose production ebbs and flows according to a strict timetable. Unlike machines, the human body changes from minute to minute, from night to day, as part of the survival strategy that has protected it from everything from wild beasts to today's high-stress environments. Blood pressure levels, resistance to disease, asthma and epileptic attacks, and the risk of many other events vary according to the time of day, while activities such as complex mental tasks, decisions and winning races are best done at specific times.

7am: Protect your immune system

The first hour after waking in the morning is the time when the immune system is at its lowest ebb, and the cells that defend the body against invading viruses and infections are at their weakest. This is why rheumatoid arthritis sufferers, for example, have the greatest pain of the day in the first hour after waking and need to adjust their medication to peak at that time. It's also why the chances of catching a cold from another member of the family are at their highest around the breakfast table - so avoid morning kisses.

Early morning is also the peak time for heart attacks, with a 70 per cent greater risk of having a heart attack between 7am and 9pm compared to the rest of the day. Even for those without a heart condition, starting the day gently is good advice. Now's the time also to drink down a big glass of water to rehydrate your liver after its night of fat processing (it processes glucose during the day) and to eat a hearty breakfast to stoke up your defences for the day.

10am: Brain power goes live

Between 10am and 1pm, your brain switches into high gear for its most active three hours of the day, making it the best time for complex mental tasks and decision-making. One of the reasons for this is that adrenalin production has its first peak of the day at 12 noon and it's this that primes the brain to work and react faster.

1pm: Siesta time

Between 1pm and 3pm comes the post-noon dip in hormone production, and the body is at its lowest daytime ebb during these two hours.

'We don't know why this is the case, but it is,' says Dr Jim Waterhouse, lecturer in physiology at Liverpool John Moores University. 'The 2pm feed, for instance, is the last one that babies give up before moving on to a 24-hour cycle. It is also the time at which, as we get older, we have a nap, and why some cultures have siestas. It is not a good time for mental challenges.'

3pm: Go for repetition

Between 3pm and 5pm, researchers have discovered, is the best time of the day for peak efficiency in repetitive jobs which do not require much concentration or memory skills. This is linked to a second spike in adrenaline around 4pm and gradually increasing body temperature.

4pm: Leap into exercise

The body reaches its highest temperature of the day around this time. With increased heat, physical performance rises and between 4pm and 7pm is the prime time for physical performance. Most athletic world records are set in the late afternoon and early evening.

5pm: Now's the time for baby

Both men and women are at their most fertile in the late afternoon.

8pm: Time for sensitivity As dusk descends, hormone production begins to fall, and the body again becomes more sensitive to allergens and other invaders of the immune system. Those prone to asthma are likely yo suffer more attacks, with midnight the peak time for sneezing. The digestive system winds down, too, getting ready for sleep - and won't appreciate a large, rich meal. And as darkness falls, levels of melatonin rise in the brain, telling it to switch off and sleep.

A suitable time for treatment

One of the spin-offs from the realisation that the body functions differently during the 24-hour cycle is that some drugs are now being made to be taken at the optimum time of day.

Such drugs - known as chronotherapeutics - are designed to be taken just before the symptoms are at their worst. Thus, rheumatoid arthritis therapies are aimed at the morning pain peak, osteoarthritis remedies in the late afternoon, and asthma at night. French studies even suggest that taking antihistamines before bed better controls hay fever, which tends to be worse first thing in the morning.

Hypertension, or high blood pressure, is another area where taking drugs at the right moment can make a substantial difference. Blood pressure surges in the early morning hours, so the most recent drugs are specially coated so that although they are taken at bedtime, they doesn't reach peak strength until around 6am when they are most needed.

Anti-cholesterol drugs known as statins are already known to work better when given in the evening because they target a cholesterol-affecting enzyme that is most active at night and other forms of therapy designed to reach high activity around cholesterol production peaks that occur during the night are also on the cards.

The latest discovery that the late afternoon is the best time for male potency may also lead to changes in fertility programmes. Though Italian researchers suggest it is the best time for spontaneous conception, it may also prove to be the best time for assisted conception for couples with infertility problems.