Have you ever noticed how a drink taken at lunch-time is much more potent than one in the evening? This is an effect of a biological clock. Biological clocks are mechanisms by which organisms can measure time, and almost all forms of life have such clocks.
Observers have been intrigued since ancient times at the fact that certain plants regularly open their leaves in daytime and close them at night. It was assumed this activity was orchestrated by the alternating sunlight and darkness.
In 1729, a French astronomer called de Mairan made the exciting discovery that such leaves stick to their routine even when kept in constant darkness.
Many biological patterns roughly follow a 24-hour schedule. These cycles are called circadian rhythms (from the Latin circa - about, and diem - day). These innate rhythms are governed by sophisticated chemical mechanisms called biological clocks.
We are all familiar with the way the time of day governs our lives. We sleep through most of the hours of darkness and are awake through most of the hours of daylight. Pulse rate and blood pressure rise sharply on awakening. Body temperature increases during the day and decreases during the night. The secretion of various hormones that control life's processes rises and falls to a circadian rhythm.
Different human circadian rhythms show maxima and minima at different times of the day. The relationship between the maxima and minima of the different rhythms is important for the smooth physiological functioning of the body.
For example, disruption of the normal pattern of the different circadian rhythms explains jetlag in travellers who cross time zones. It takes days, or even weeks, for all the circadian rhythms to readjust to their new external time. The internal desynchronisation encountered during the readjustment is experienced as jet lag. This is also important for people on shift work. A change in shifts can be equivalent to crossing several time zones.
Why do living organisms do most things on a daily cyclical basis? Monthly and seasonal cycles are also widespread in nature. These cycles arose because life evolved on a planet that displays marked and regular temporal fluctuations in environmental characteristics such as light, temperature, electromagnetic and gravitational forces.
The regular fluctuations in environmental conditions are caused by specific planetary cycles, like the daily rotation of the Earth on its axis (day/night cycle), the monthly revolution of the Moon around the Earth, and the annual revolution of the Earth around the Sun (tidal and seasonal cycle).
Organisms that can modify their physiology and behaviour in tune with these daily, monthly and annual cycles have an advantage, hence the evolution of biological clocks to measure time.
The treatment of disease is one practical application of our understanding of the role of cycles in biology. The abnormalities accompanying almost every disease, e.g., asthma, arthritis, allergy, heart disease and cancer, present themselves with circadian rhythm. Therapy is improved and toxic effects reduced when drugs are administered at carefully selected times of the day - chronotherapy.
The pharmaceutical industry makes a lot of money out of drugs that control allergic symptoms, for example, antihistamines and decongestants. Antihistamines generally have a sedative effect and it is therefore best to take them in the evening. Decongestants, which have a stimulator effect, should be avoided at night and taken during the day.
Asthma affects one in every 20 people and its symptoms operate to a strict circadian rhythm. The distressing symptom occurs when the smooth muscle surrounding the airways contracts, making it difficult to breath. Most asthma attacks take place between two and six o'clock in the morning.
Airway size and patterns of breathing change in a rhythmic fashion throughout the day in all people. Airways are open widest during the day, but there is a reduction in the air flow after midnight, particularly between 2 and 6 a.m.
This normal fluctuation can be greatly accentuated in response to internal and external stimulation. These stimuli include allergens in the bedroom, mucus retention during sleep, cooling of airways caused by breathing through the mouth, circadian patterns in muscle and nerve tone and in the circulation of hormones.
Drug treatment of asthma, e.g., the bronchodilator preparation for nocturnal asthma, should be designed to anticipate the temporal nature of the attack.
Cardiovascular disease is the single biggest killer of adults and is heavily influenced by circadian rhythms. The immediate cause of a heart problem is restricted flow of blood to the organ: ischaemia. There may be no obvious symptoms of a heart condition, or it may cause angina (chest pain), or, most severely, a heart attack may occur.
Several large studies have shown that ischaemia and heart attacks are much more common in the morning than at other times of the day. Drugs treatment for heart disease should clearly take this marked circadian aspect into account.
William Reville is a senior lecturer in biochemistry at UCC.