Life on earth has evolved with the ability to cope with cyclical changes in the environment. The length of these environmental cycles is determined by our planet’s rotational period (day-night cycle) as well as the period of its revolution around the sun (e.g., seasonal cycles). The survival of an organism depends on its ability to initiate physiological (e.g., body temperature, hormonal secretions) and behavioral events (e.g., activity, migration, hibernation) at the appropriate point in the environmental temporal cycle. In order to do so, an organism needs an effective internal timekeeping mechanism. Biological timekeeping ensures that this occurs by anticipating important events in the environment and keeping track of the passage of time. The biological clock, the body’s internal chronometer, is responsible for regulating and maintaining an internal temporal structure that is stable and synchronized with the appropriate environmental time cycle. Biological rhythms are the output of the biological clock, and their cycle can range from a few milliseconds to days or even months.
The most commonly observed biological rhythms are circadian rhythms. The word circadian comes from two Latin words, “circa,” which means “about,” and “dies,” which means “a day.” Circadian rhythms are the output of the circadian oscillator or clock. We can measure the endogenous period of the circadian oscillator by observing its output rhythms under constant environmental conditions. The period of the circadian oscillator when measured under these conditions is called the free running period (FRP). It is generally a little longer or a little shorter than 24 hours and will remain the same under constant conditions. The free running period will also remain unchanged when measured at different ambient temperatures.
One of the functions of the circadian oscillator is to anticipate events in the environment and estimate the passage of time so that key physiological and behavioral events occur at the appropriate point in the environmental time cycle. To achieve this, the circadian oscillator needs to be synchronized with the environmental time cycle. Since the endogenous period of the circadian oscillator is a little shorter or a little longer than 24 hours, it has to be either delayed or advanced to match the 24-hour period of a natural environmental cycle. There are many different environmental factors such as temperature, humidity, light, social variables, and food availability that oscillate with a period of 24 hours and serve as effective synchronizers. The environmental stimulus that synchronizes the circadian oscillator is called the entraining agent or zeitgeber. Research has demonstrated that the 24-hour light-dark cycle is the most powerful entraining stimulus. Exposure to light can cause the circadian rhythm to shift appropriately (i.e., advance or delay) to match the period of the entraining environmental stimulus.
In the 21st century it has become very important for us to develop an understanding of the circadian system and its functions. The need for constant availability of essential services in modern society requires many in the workforce to reverse their normal diurnal sleep/ wake schedule as in the case of rotating or night-shift work. Such schedules result in an individual’s physiological and behavioral rhythms being initiated at inappropriate points in the environmental time cycle. This can exact a substantial cost in terms of health and degraded performance. Understanding the circadian system, in particular its entraining mechanism, can provide us with useful solutions to these problems.
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