Our bodies are subject to hourly, daily, and even longer natural rhythm cycles. These rhythms dictate when 11e are most vulnerable to illness when a baby is most likely to be conceived and born, and how we respond to drugs. Developments in our understanding of the body’s daily rhythm – the circadian cycle – have suggested new possibilities for improving health by getting in tune with our natural body clock.
All bodily functions rhythmically vary over time. Some regular body rhythms, such as the beating of the heart and the firing of neurons in the brain, have cycles of less than a second. Others are much longer: for example, there is a yearly cycle in sexual activity, which leads to more babies being born in late summer than at any other time. Indeed, in the USA, government statistics show that over the last 60 years, more babies were born in August than in any other month. It is thought that the increased sexual activity in late autumn is due to men having higher testosterone levels. No- one is quite sure why this is, although it may be linked to the changing amount of daylight. Seasonal cycles are most pronounced in other animals, where the annual routine of hibernation, breeding, nurturing, and migration dictate the essential activity patterns in an animal’s life.
The circadian clock
In humans, by far, the most pronounced body rhythm is the circadian cycle, which is controlled by the brain. Circadian m.ea ns ‘a round a day’ and describes how physiological properties, such as body temperature and blood pressure, change in a regular cycle of 24- 25 hours. The circadian cycle runs (more or less) alongside our daily cycle of sleeping and waking. It is regulated by a ‘biological clock,’ which responds to external cues, the most important of which is the level of light.
The circadian clock is situated in the suprachiasmatic nuclei (SCN), a patch of around 100,000 cells located in the hypothalamus at the base of the brain. The SCN lies above the optic chiasma, a major nerve junction linking the eyes to the brain, and connections between the optic nerves and the SCN a.How cells within the SCN respond to rising or falling light levels. Onward connections between the SC N and the hypothalamus tune physiological functions accordingly. Thus, after dawn, rising light levels tell the SCN to wake up the body. At night, falling light levels cause the SCN to activate the nearby pineal gland, resulting in the production of melatonin – the ‘darkness hormone ‘– which helps prepare the body for sleep.
Rest and activity
The circadian rhythm also includes many short er cycles. One of the most familiar is the rest-activity cycle, which lasts between 90 and 120 minutes. You may have noticed that although you start the day awake and finish it asleep, you don’t just become gradually more tired as the day wears on. Many of us experience a dip in energy in the mid-afternoon period. Although you may feel worn out \ when you get home from work in the late afternoon or early evening, it is quite common to experience a ‘second wind ‘between 8 pm and 10 m – in fact, most people find it almost impossible to fall asleep at this time of the evening. The Israeli scientist Peretz Lav i.e., demonstrated that there are sleep ‘gates’ that open every 90 minutes or so. In his experiments, he asked people to try to stay awake or fall asleep at 20-minute intervals over 24 hours and, as a result, found this marked rest-activity cycle in operation.
Blood pressure variation
Circadian rhythms have important implications for health and well-being. Blood pressure, for example, can be up to 20 percent lower in the morning than in the afternoon. If you have high blood pressure, it may be missed if the doctor always measures your blood pressure in the morning. On the other hand, if it is measured in the afternoon, the doctor may think you have high blood pressure when, in fact, you do not. Also, suppose you are taking medication for high blood pressure. In that case, it is harder for your doctor to assess how well the treatment is working if the natural circadian variation is not taken into account.
The daily variation in physiological rhythms also means that many illnesses are at their worst – or are more likely to strike – at certain times of the day. For example, heart attacks are two to three times more likely to occur in the morning than later in the day, partly because blood pressure is rapidly rising. Bloo d is thicker in the morning, making it more likely to form a clot in the blood vessels leading to the brain (causing a stroke) or the heart.
Seasonal Affective Disorder
Every autumn, some people feel a familiar sense of dread. They find it increasingly difficult to get up in the morning and often crave comfort foods, such as cheese on toast and chocolate. As a result, they may start to gain weight, adding to their misery. By the middle of winter, they are feeling hassled and desperate. These are all typical symptoms of seasonal affective disorder, also known as SAD, a form of depression triggered by falling light levels. However, in SAD, the pattern of symptoms is different from those in clinical depression, which may involve waking early and loss of appetite. SAD depression usually lifts with the coming of spring. In the winter, the symptoms are treatable with antidepressants or light therapy, which involves being exposed to bright artificial light for several hours a day to make up for the lack of sunlight. However, for some people, the only permanent solution to SAD is a move to a sunnier climate.
Body rhythms and medicine
Because this variety of illnesses will vary during 24 hours, the need for medication varies, too. What is more, the way the body handles drugs changes over the day. In the case of medications taken orally, variations in blood flow and stomach acidity will affect the amount of drug that is absorbed, while the rhythms of kidney and liver activity determine how long the drug stays in the body before being excreted. This has implications for side effects as well as for the drug’s effectiveness. For example, if you take aspirin in the morning, it is more likely to cause stomach irritation than aspirin taken at night. When your doctor instructs you to take medicine in three equal doses three times a day, this ass um.es that your body’s responses remain the same all day long; in fact, you will probably absorb a different amount of the drug each time. Ideally, doses throughout the day should be adjusted to take account of your natural body rhythms – this is the principle behind Chrono medicine.
FIND YOUR TEMPERATURE RHYTHM
The common belief that 37° Celsius is normal body temperature is inaccurate – this figure is actually an average. Body temperature, in fact, ranges from about 36.5°C to 37.5°C throughout the day. It is at its lowest early in the morning, two to three hours before you normally wake up, and rises through the morning and early afternoon to reach a plateau in mid-afternoon. Then it dips a little and increases again to peak at around 7 pm. A temperature of, say, 37°C might indicate that you had a mild fever if measured at dawn but would be considered normal later in the day.
- You can track your daily temperature rhythm. Take your temperature first thing in the morning, before you even get out of bed, and after that at two-hour intervals throughout the day until bedtime. Do this for several days and plot the temperatures onto a graph like the one shown above to see if you can identify a regular pattern to the rhythm. If you are unwell (with a bad cold or flu, for example) and find that your morning temperature is less than it was the evening before, remember that this may not necessarily mean that you are well again. It could just be the natural variation of your normal temperature rhythm – so you should carry on taking care of yourself until you are sure that you have recovered fully.
