By Gary Hayden

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.

We saw this in the first two parts of The Body Clock.

Many of our physiological functions vary depending on the time of day.

Body temperature, blood pressure, heartbeat and hormone levels all show daily changes. Our moods, levels of alertness, physical performance and mental abilities also dip and peak at predictable times.

A time for everything
Because of these rhythms, our ability to perform mental and physical tasks varies throughout the day. There are optimum times for carrying out all kinds of activities.

Early evening is a great time to exercise. Athletic performance peaks between about 6pm and 8pm when our body temperature is highest, our muscles are strongest and we are at our most flexible. It is no surprise then, that Olympic records are often broken at this time.

In contrast, the body is most primed for sex early in the morning, just after we wake up, when men's testosterone levels and women's oestrogen levels are high.

Despite this, the preferred time for making love is 10pm - for most of us, a more convenient time.

Our ability to reason and solve complex problems peaks at around midday, as does our ability to concentrate. So it is a good idea to get the day's brain-work done before lunch, if possible.

Research has shown that even the firmness of a handshake varies according to the time of day. Grip strength peaks at the same time as body temperature, during the early evening.

Studies have shown that the intensity of pain also varies throughout the day, and that different kinds of pain peak at different times. The reasons for this are still not clear.

But toothache is lowest during the mid-afternoon (something to bear in mind when making dental appointments) and most painful around 10pm. Migraine, on the other hand, hits hardest at breakfast time.

Even complex physiological processes such as childbirth show circadian rhythms.

Labour pains are most likely to begin at night, and delivery occurs most often during the early hours.

Our bodily rhythms make us more susceptible to certain illnesses at specific times of day. For example, an asthma attack is several hundred times more likely to occur at night than it is during the day, and is most likely to occur at 4am.

The risk of a heart attack is highest between 6am and midday: the peak time for both blood pressure and heart rate.

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