Biological clock refers to a timing mechanism that operates in living things. Biological clocks control the rhythms of functions and processes in organisms. They keep accurate time during each 24 hours and over days, weeks, months, and even years. Biological clocks keep the activities of living things in harmony with regular changes in the surroundings.
Birds migrate, fish spawn, and flowers blossom on schedules that are set by their built-in clocks. In human beings, biological clocks time periods of sleep and wakefulness and of body activities. The science that deals with the study of biological clocks and rhythms is called chronobiology.
No one is certain where biological clocks are located or how they work. Experiments indicate every living thing inherits timing mechanisms. Most scientists believe biological clocks occur in several forms and regulate processes in such simple structures as cells, as well as complex organs and organ systems. Research indicates that the pineal gland and the hypothalamus in the brain may be the master clocks in animals.
Importance of biological clocks
Biological clocks keep track of cyclic variations in the environment, including day and night, movements of the ocean tides, phases of the moon, and seasons of the year. Most, if not all, living things have internal cycles–called biological rhythms–that are controlled by biological clocks. The biological rhythms of each particular species are timed to enable the organism to efficiently meet the demands of its environment. Biological rhythms continue on schedule even in laboratories where the organism is shielded from all evidence of passing time and of outside change. But the rhythms can be shifted–and the biological clock reset–by changing the time at which the organism gets light or by changing other critical time cues from the environment.
Daily rhythms.
Many biological rhythms are based on a day-night cycle. They are called circadian rhythms because they occur about every 24 hours. Circadian comes from Latin words that mean about a day. For most living things, the day-night cycle is broken into periods of activity and periods of rest. But these periods do not occur at the same time of day for all living things. Human beings are most active during the day and rest at night. Apes, bees, butterflies, monkeys, and many other kinds of animals also follow this schedule. On the other hand, bats, cats, moths, owls, rats, and others are active at night. The genetically inherited traits of biological clocks in each species set the schedule.
Plants also show daily rhythms. For example, they raise their leaves in the day and lower them at night. These rhythmic changes, called sleep movements, continue even when the plants are kept in caves or in other places where light and temperature do not change.
Other rhythms.
Fiddler crabs and other seashore animals show complex rhythms. The skin of fiddler crabs normally darkens at dawn and gets pale at dusk. Their running activity adjusts to the tides, which rise and fall about 50 minutes later each day. Fiddler crabs kept in constant darkness in laboratories continue to change color rhythmically, as if responding to the tides of their home beach. But when they are moved to a beach that has different tidal times, they adjust activities to the new tides. Their biological clocks have been automatically reset.
Many living things, including the grunion, a small fish found along the California coast, have monthly or semimonthly breeding rhythms. From February to September at maximum high tide, every 14.8 days, grunions ride a wave to shore. The wave recedes, the females drop their eggs in the wet sand, and the males fertilize the eggs. The next wave carries the grunions back into the ocean, but the eggs remain on the beach. At the next high tide–14.8 days later–a wave comes in, breaks the eggs, and carries the young fish out to sea.
Biological clocks set the schedules for yearly rhythms in living things. They control the sprouting of seeds and the hibernation and migration of birds and other animals. These clocks also seem important in helping birds, fishes, crustaceans, and insects to navigate. The clocks, used in conjunction with the sun, moon, and stars, help them to correct continuously for the earth’s rotation, and stay on the proper course. Biological clocks also coordinate breeding cycles in animals to seasonal changes in the amount of daylight. Some species of animals mate in the fall, when the period of darkness exceeds the period of light. Other species breed in the spring, when the period of light is greater than that of darkness.
Biological clocks in people
Biological clocks in people work on schedules essential to life and health. Human beings have daily, weekly, monthly, and seasonal biological rhythms. The level of hormones and other chemicals in the blood varies dramatically over each of these time periods. Most vital body processes have a circadian rhythm. The activities of the cells, glands, and organ systems are coordinated with one another and with the day-night rhythm of the environment.
The rate at which the body processes work varies rhythmically throughout the day and night. For example, in most people who are active during the daytime, body temperature varies about three degrees during a 24-hour period. The temperature is lowest during sleep and greatest during the afternoon and early evening.
You become most aware of the biological timing system when you fly by airplane to a different time zone. A flight from Chicago to London that leaves Chicago in late afternoon will arrive when Londoners are just starting their day. Your rhythmic system will still be working on Chicago time. By London time, you will have insomnia at night and be sleepy during the day. Your biological clock will reset itself, but it may take several days. As a result, your body functions are out of rhythm, your efficiency drops, and you feel tired. This is called “jet lag” or “jet exhaustion.” Many workers who switch from the day shift to the night shift also experience jet-lag symptoms. Scientists hope to find so-called chronobiotics, medications that can quickly reset the body’s biological rhythms.
The symptoms or occurrence of many diseases follows biological rhythms. For example, cerebral hemorrhages are most common late in the evening. Most heart attacks occur in the morning. Most people who suffer from asthma feel worse in the evening and overnight. The study of influences that biological rhythms have on human diseases is called chronopathology.
Biological rhythms also influence the effects medications have on illnesses. Many medications, such as those used to treat allergies, arthritis, cancer, and heart disease, are strongly affected by circadian rhythms. The branch of chronobiology that deals with the study of biological rhythms and medications is called chronopharmacology. Greater knowledge of biological rhythms in the treatment of diseases could result in important changes in the practice of medicine.
