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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.
Empirically, it is an ultradian rhythm of approximately 90 minutes (80–120 minutes [2]) characterized by different levels of excitement and rest. The cycle is mediated by the human biological clock. It is most readily observed in stages of sleep, for example, rapid eye movement sleep (REM) and the delta activity cycle.
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 ...
Phase response curves for light and for melatonin administration. In humans and animals, there is a regulatory system that governs the phase relationship of an organism's internal circadian clock to a regular periodicity in the external environment (usually governed by the solar day).
"The Brutalist," starring Adrien Brody, Felicity Jones, and Guy Pearce, is 3 hours and 35 minutes.. Over the last few years, blockbusters have become longer and longer. Three-hour movies are ...
In horology, a complication is any feature of a timepiece beyond the display of hours, minutes and seconds. A timepiece indicating only hours, minutes and seconds is known as a simple movement. Common complications include date or day-of-the-week indicators, alarms, chronographs (stopwatches), and automatic winding mechanisms.
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