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The basic rest–activity cycle (BRAC) is a physiological arousal mechanism in humans proposed by Nathaniel Kleitman, [1] hypothesized to occur during both sleep and wakefulness. Empirically, it is an ultradian rhythm of approximately 90 minutes (80–120 minutes [ 2 ] ) characterized by different levels of excitement and rest.
In humans, this cycle takes 70 to 110 minutes (90 ± 20 minutes). [1] Within the sleep of adults and infants there are cyclic fluctuations between quiet and active sleep. These fluctuations may persist during wakefulness as rest-activity cycles but are less easily discerned. [2]
Services that alternate between high and low utilization in a daily cycle are described as being diurnal. Many websites have the most users during the day and little utilization at night, or vice versa. Operations planners can use this cycle to plan, for example, maintenance that needs to be done when there are fewer users on the web site. [13]
In schedules involving night shifts and fragmentation of duty periods through the entire day, a sort of polyphasic sleeping schedule was studied. Subjects were to perform two hours of activity followed by four hours of rest (sleep allowed), this was repeated four times throughout the 24-hour day.
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.
Local Street Fair or Carnival. If the local street fair or carnival is in town, bring your date and head on over. For under $25, you might be able to snag two tickets and enjoy a few cheap games ...
The kiwi is a family of nocturnal birds endemic to New Zealand.. While it is difficult to say which came first, nocturnality or diurnality, a hypothesis in evolutionary biology, the nocturnal bottleneck theory, postulates that in the Mesozoic, many ancestors of modern-day mammals evolved nocturnal characteristics in order to avoid contact with the numerous diurnal predators. [3]
For example, the occurrence of midnight sun in the Arctic induces changes to planktonic life that would normally perform DVM with a 24-hour night and day cycle. In the summers of the Arctic the Earth's north pole is directed toward the sun creating longer days and at the high latitude continuous day light for more than 24-hours. [10]