A technological approach to helping people relax is through the use of biofeedback. On a simple level this could involve employing a machine to register blood pressure. All the person has to do is make the instrument register a lower reading, because stress raises blood pressure whereas relaxation lowers it. Quite how a lowered pressure is achieved by ‘thinking’ about it is not clear but the person sees he or she is achieving the desired result, and this behaviour can be learned and eventually performed even without the aid of the instrument. Remember that our bodily stress mechanisms are valuable to us because they protect us in dangerous situations; but it is dangerous in a different way to leave them switched on for pro-longed periods – that is why we have to learn to relax.


We are normally awake during the hours of daylight but want to sleep during darkness. Women menstruate every 28 days. Dogs usually come into heat twice every 12 months. These are just a few examples of biorhythms. Many biorhythms have a periodicity of a day and are known as circadian rhythms (from the Latin circa about, dies day). A circadian rhythm is defined as a regular, repetitive, quantifiable physiological change having a period of about 24 hours. Some biorhythms are much shorter. To a certain degree, for example, the actions of the brain hemi-spheres alternate with a periodicity of one and a half hours. This means that at one moment we are apt to think in a more logicalanalytical way, whereas 90 minutes later there is a preference for thinking in images. The significance of this phenomenon has not yet been established, however. Other biorhythms are much longer: for example, deer have an annual repro-ductive cycle and this is known as a circaannual rhythm. There is statistical evidence that heart attacks have a preponderance in January in the Western World, whereas in countries below the equator this prevalence is highest in July – when it is winter there. Presumably this strange rhythm is partly caused by a change in blood coagulation factors. The human menstrual cycle is a biorhythmic cycle of approximately four weeks.

One of the oldest established human biorhythms is that of body temperature. It was first observed in 1873 that the body’s temperature rises in the morning and remains elevated throughout the day, reaching a peak in the middle of the afternoon. This pattern is not influenced in the broadest sense by fasting or other activities. There are also several hormonal circadian rhythms. The growth hormone and the steroid hormone Cortisol peak just before we rise in the morning.

The importance of knowing when the blood levels of these hormones rise and fall is paramount to doctors when they are investigating conditions such as Addi-son’s disease , in which sensitive measurements of Cortisol levels are critical in diagnosis. Another well-established circadian rhythm is that of the enzyme amylase in saliva. It peaks between the hours of midday and early evening and is not influenced if we fast or change our mealtimes.


Biorhythms are not merely scientific curiosities but are important to medicine, and especially occupational health. At the present time testing and using therapeutic chemicals generally does not take into account the time of the day a drug is administered. In future the research on time-dependent effects of drugs may result in prescriptions that do not only mention the total amount of medicine that should be taken, but also the time of day at which it will have its greatest effect. Certain cytotoxic, or cell-killing drugs -used for the treatment of cancer – work most ef-ficiently at night.

Biorhythms occur at all levels of organization in the human body from the production of enzymes within cells to overall mood and intellectual performance. They are not learned but are programmed into our genetic material. Some scientists have proposed that biorhythms are entirely dependent on external (exogenous) factors such as magnetism or radiation, but the majority now accept the idea that biorhythms are controlled within the body, although external factors can influence these internal (endogenous) controls. The external influencing factors are known as syn-chronizers or entraining agents. The most important entraining agent for plants, animals and humans is light, but even totally blind people can maintain precise circadian rhythms, provided they are in social contact with sighted people.

In the absence of any kind of synchronizers or entraining agents, circadian (and other periodic) rhythms return to their natural interval that relies totally on the endogenous ‘biological clock’. Under such circumstances the biorhythm is said to be free-running. The natural free-run period of a circadian rhythm may be 25 hours in one individual and 23 hours in another. The importance of free-running is that it enables plants, animals and humans to adjust themselves to the changing seasons, which are principally changes in the amount of daylight and darkness not exactly tied to 24 hours. Much is still unknown about what actually sets the timing and ultimately regulates the endogenous biological clock, however. It has become clear that the pineal body in the brain plays an important role in biological time regulation of animals. This tiny organ appears to be able to detect light, even if the animal is blind (and in this regard it is interesting to know the pineal body was formerly called the ‘third eye’). At night the hormone melatonin is secreted, which immediately or via another centre in the brain – called the nucleus supra-chiasmaticus – synchronizes all the bodily functions, like a sort of ‘master clock’.


Biological rhythms can become a problem in the modern world. A long jet flight could result in as many as six time zones being crossed in less than six hours. When we travel in this way our biological rhythms can be put out of phase by as much as seven hours. The result is ‘jet lag’. Over the next seven or eight days the biorhythms free-run until they catch up with the synchronizers such as daylight at the destination. The immediate problem is that different bio-rhythms normally have precise relationships to each other but when allowed to free-run they do so at different intervals. That is why people tend to feel unwell when they are suffering from jet lag. Many experts advise that the jet-setter should try to fall in with new circadian rhythms as quickly as possible. The significance of jet lag is even more important to the pilots and crew who do the flying. Scientists dis-covered that it took pilots about five days to resyn-chronize to their new environments after an eight-hour time change.

It is not only international travel that creates havoc with our biorhythms. A much more common ex-perience is that of shift work. Studies have shown that it is virtually impossible to reverse the normal cir-cadian pattern (’turning day into night’) unless workers are completely isolated from light, sound and other cues that people rely on when they are working normal hours. Obviously this would be socially unac-ceptable to most of us. The result is that shift workers – especially those on night shifts – can only partly adapt. The problem is further complicated because the nature of the job also has an effect. The more boring the job, the worse the relative productivity of the nightshift operator, compared with the dayshift equivalent. Some people are better suited to night-working than others, however. A study of computer operators showed that people can be divided into morning, afternoon and evening types. In general ‘morning’ people are least suited to nightshifts whereas people most active in the evening are best suited to it. It is evident, however, that nightshift work, and in particular repetitive work should be carried out only when it is essential.

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