Biological clocks are internal mechanisms that help humans, animals, and other living organisms measure time and regulate the rhythms of their bodies and activities. Most commonly, these clocks tell us when to eat, sleep, rest, become active, and mate. Biological clocks allow organisms to determine internally, at any given moment, the actual time of day, month, or year. The internal mechanisms are triggered by fluctuations in environmental variables like changes in daylight or temperature. These internal timekeeping devices have evolved to interrelate with the daily, lunar, and seasonal changes. Environmental rhythms are predictable and set their course on planetary cycles. For example, as the earth spins on its axis, we experience day and night patterns. When the moon orbits the earth, we experience lunar phases and daily tidal changes. The predictable seasonal changes occur during the yearlong travels of the earth around the sun. Even absent human-made calendars and clocks, living organisms detect these environmental variables and naturally modify their behavior consistent to these variables.
Body time is controlled by the suprachiasmatic nucleus, or SCN. This cluster of cells resides within the hypothalamus, which is a tiny portion of the brain that forms on the bottom of the left ventricle and regulates many basic bodily functions, such as body temperature. The ability to synchronize biorhythms to the day and night cycles of the earth’s 24-hour-long rotation around its axis provides evidence that this internal mechanism exists and responds to environmental change. The most common examples of biological rhythms for humans include sleeping, waking, eating, urination, and physical and mental performance and alertness. These patterns of behavior show that organisms behave as if they are independently capable of determining time.
The scientists who study biological clocks are called chronobiologists; they believe the biological clock begins functioning even before humans are born and that it develops as they grow. At 7 months of age, human babies have developed sleep patterns that are free-running, meaning that their sleep cycles are not regulated by external cues such as the rising or setting of the sun. Chronobiologists have also discovered that biological rhythms send messages to the brain, telling people what activities to perform. Because the urinary biological rhythms take longer to develop in babies, they wear diapers until they have mastered the rhythm, sometimes until they are 3 or 4 years old. As children grow, the biological clock matures and begins to slow urinary frequency during sleep hours, freeing older children from the urge to urinate and allowing them nights of uninterrupted sleep. The circadian or daily rhythms of 10-year-olds are disrupted by their high energy levels. Concomitantly, adolescents hit growth spurts and require more sleep than they did when they were younger. When they reach their 20s, they generally settle into a routine 24-hour day cycle. As energy levels fall for the elderly, their sleep cycles and circadian patterns are again disrupted.
Biological rhythmicity has also been seen in research studies involving groups of plants, animals, and microorganisms. In the late 19th century, plant physiologists researching circadian rhythm found that if a plant was removed from its natural habitat and was confined to a laboratory and subjected to disturbances in its daily cycles of day and night, the plant would maintain its primary biorhythms. Many plants that exhibit daily rhythms in which they raise and lower their leaves to correlate to the time of day or night will continue to exhibit similar patterns even after the source of light is disrupted. For example, if a plant normally lifts is leaves upward during daylight hours, it will continue to do so even if placed in a laboratory and removed from its light source. These plants slowly adapt to the new environment provided in the controlled study and can eventually reverse their normal daytime/ nighttime routines. They can later even revert successfully to their natural habitat and primary biorhythms when returned to their native habitat. Laboratory experiments have consistently shown this extraordinary stability of biological rhythms. Daily rhythms for a wide variety of single-cell organisms, mammals, and plants also remain unchanged when they confront constant light or dark.
A strong sense of time enables organisms to most fully explore and participate in the portions of the day in which their activities will be most productive (eating, mating, sleeping, hunting, migrating, etc.). When an animal is relocated to a geographic area where the environmental cycle is greatly different, the animal’s clock maintains synchronicity with its original environment. Similarly, if humans are rapidly translocated a great distance and experience significant time zone disruptions, they will also maintain synchronicity. Most humans and animals will slowly readjust after this period of initial disruption. This reaction to loss of time is a phenomenon commonly called jet lag, and many people describe physical effects of fatigue, lowered productivity, and decreased alertness until their internal mechanisms readjust.
Fluctuations occur at every level, from cellular to complete physiological activities, in which chemical, behavioral, and physiological responses are evidently correlated to the environment. Many biological rhythms correlate to periodic planetary changes, also referred to as geophysical correlates (ocean tides, amount of sunlight in a day, the changing months and years, and the seasonal differences). Some fluctuations or biorhythms do not have geophysical or external correlates, such as the biological rhythm of a heartbeat or a respiratory rate.
Debra M. Lucas
See also Cryonics; DNA; Hibernation; Life Cycle; Longevity; Metamorphosis, Insect
Dunlap, J., Loros, J., & DeCoursey, P. (Eds.). (2004).
Chronobiology: Biological timekeeping. Sunderland, MA: Sinauer Associates.
Koukkari, W., & Sothern, R. (2006). Introducing biological rhythms: A primer on the temporal organization of life, with implications for health, society, reproduction and the natural environment. New York: Springer.