Feb 24, 2008

Overview of circadian rhythms

You certainly don't need to be an expert on human physiology to work shiftwork, but understanding a bit of the science can be both interesting and helpful if you're a shiftworker. So with this and future posts we'll be bringing you information to help you do just that.

The daily light-dark cycle governs rhythmic changes in the behavior and/or physiology of most species. Studies have found that these changes are governed by a biological clock, which in mammals is located in two brain areas called the suprachiasmatic nuclei. The circadian cycles established by this clock occur throughout nature and have a period of approximately 24 hours.

In addition, these circadian cycles can be synchronized to external time signals but also can persist in the absence of such signals. Studies have found that the internal clock consists of an array of genes and the protein products they encode, which regulate various physiological processes throughout the body. Disruptions of the biological rhythms can impair the health and well-being of the organism.

One of the most dramatic features of the world in which we live is the cycle of day and night. Correspondingly, almost all species exhibit daily changes in their behavior and/or physiology. These daily rhythms are not simply a response to the 24-hour changes in the physical environment imposed by the earth turning on its axis but, instead, arise from a timekeeping system within the organism.

This timekeeping system, or biological “clock,” allows the organism to anticipate and prepare for the changes in the physical environment that are associated with day and night, thereby ensuring that the organism will “do the right thing” at the right time of the day. The biological clock also provides internal temporal organization and ensures that internal changes take place in coordination with one another.

The synchrony of an organism with both its external and internal environments is critical to the organism’s well-being and survival; a lack of synchrony between the organism and the external environment may lead to the individual’s immediate demise. For example, if a nocturnal rodent were to venture from its burrow during broad daylight, the rodent would be exceptionally easy prey for other animals. Similarly, a lack of synchrony within the internal environment might lead to health problems in the individual, such as those associated with jet lag, shift work, and the accompanying sleep loss (e.g., impaired cognitive function, altered hormonal function, and gastrointestinal complaints).

The mechanisms underlying the biological timekeeping systems and the potential consequences of their failure are among the issues addressed by researchers in the field of chronobiology.
In its broadest sense, chronobiology encompasses all research areas focusing on biological timing, including high-frequency cycles (e.g., hormone secretion occurring in distinct pulses throughout the day), daily cycles (e.g., activity and rest cycles), and monthly or annual cycles (e.g., reproductive cycles in some species).

Among these interrelated areas of chronobiology, we'll focus on one frequency domain-the daily cycles known as circadian rhythms.

The term “circadian” derives from the Latin phrase “circa diem,” which means
“about a day.”

Although virtually all life forms- including bacteria, fungi, plants, fruit flies, fish, mice, and humans-exhibit circadian rhythms, this review is primarily limited to the mammalian system. Other animals are discussed only in cases in which they have contributed to the understanding of the mammalian system, particularly in studies of the molecular genetic makeup of the time-keeping system.

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By Martha Hotz Vitaterna, Ph.D., Joseph S. Takahashi, Ph.D., and Fred W. Turek, Ph.D.

MARTHA HOTZ VITATERNA, PH.D., is a senior research associate in the Center for Functional Genomics, Northwestern University, Evanston, Illinois. JOSEPH S. TAKAHASHI, PH.D., is the director of the Center for Functional Genomics, the Walter and Mary E. Glass Professor in the Department of Neurobiology and Physiology, and an investigator at the Howard Hughes Medical Institute, Northwestern University, Evanston, Illinois. FRED W. TUREK, PH.D., is the director of the Center for Sleep and Circadian Biology and is the Charles T. and Emma H. Morrison Professor in the Department of Neurobiology and Physiology, Northwestern University, Evanston, Illinois.

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