Academic journal article Alcohol Research

Overview of Circadian Rhythms

Academic journal article Alcohol Research

Overview of Circadian Rhythms

Article excerpt

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. KEY WORDS: circadian rhythm; time of day; biological regulation; biological adaptation; temperature; light; hypothalamus; neural cell; gene expression; mutagenesis; sleep disorder; physiological AODE (effects of alcohol or other drug use, abuse , and dependence)

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 "dock," 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 dock 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 wellbeing 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. [1] In its broadest sense, chronobiology encompasses all research areas focusing on biological timing, including high frequency cydes (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, this article focuses 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 un derstanding of the mammalian system, particularly in studies of the molecular genetic makeup of the timekeeping system. (For comparative discussions of other nonmammalian model systems that have contributed to the depth of understanding of circadian rhythmicity in mammals, the reader is referred to Wager-Smith and Kay 2000.) Overall, this article has the following major objectives: (1) to provide a highly selective historical overview of the field, (2) to review characteristic properties of circadian rhythms, (3) to define the structural components and the molecular genetic mechanisms comprising the biological clock, and (4) to explore the health effects of biological rhythms. …

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