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A circadian rhythm (/ s ər ˈ k eɪ d i ə n /), or circadian cycle, is a natural oscillation that repeats roughly every 24 hours. Circadian rhythms can refer to any process that originates within an organism (i.e., endogenous ) and responds to the environment (is entrained by the environment).
In vertebrates, the master circadian clock is contained within the suprachiasmatic nucleus (SCN), a bilateral nerve cluster of about 20,000 neurons. [10] [11] The SCN itself is located in the hypothalamus, a small region of the brain situated directly above the optic chiasm, where it receives input from specialized photosensitive ganglion cells in the retina via the retinohypothalamic tract.
The physiological changes that follow these clocks are known as circadian rhythms. Because the endogenous period of these rhythms are approximately but not exactly 24 hours, these rhythms must be reset by external cues to synchronize with the daily cycles in the environment. [1] This process is called entrainment. One of the most important cues ...
All of these are examples of circadian rhythm disorders, which are problems with the alignment between your sleep-wake cycle and your responsibilities for the day—like going to work or school ...
Together, these data suggest a model whereby the SCN maintains control across the body by synchronizing "slave oscillators," which exhibit their own near-24-hour rhythms and control circadian phenomena in local tissue. [11] The SCN receives input from specialized photosensitive ganglion cells in the retina via the retinohypothalamic tract. [12]
In single celled organisms, circadian rhythms are believed to be generated without the use of a TTFL, but rather with a 3 protein complex called the KaiABC Complex. The mechanism of entrainment in this system is known to be controlled by various proteins. [11]
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Circadian rhythms, controlled by oscillating genes, have a period of approximately 24 hours. For example, plant leaves opening and closing at different times of the day or the sleep-wake schedule of animals can all include circadian rhythms.