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A constant routine protocol is a common method used in human circadian rhythm research to study internally generated, or endogenous, circadian rhythms without the effect of external, or exogenous, influences. In the method, subjects are kept in constant conditions for at least 24 hours. [1]
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).
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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.
A circadian rhythm is an entrainable, endogenous, biological activity that has a period of roughly twenty-four hours. This internal time-keeping mechanism is centralized in the suprachiasmatic nucleus (SCN) of humans, and allows for the internal physiological mechanisms underlying sleep and alertness to become synchronized to external environmental cues, like the light-dark cycle. [4]
Misdiagnosis of circadian rhythm sleep disorders as psychiatric conditions causes considerable distress to patients and their families, and leads to some patients being inappropriately prescribed psychoactive drugs. For many patients, diagnosis of DSPD is itself a life-changing breakthrough.
The continuous mode is when the circadian rhythm is continuously adjusted by the environment, usually by constant light. Two properties, the free-running period of an organism, and the phase response curve, are the main pieces of information needed to investigate individual entrainment. There are also limits to entrainment.
Early research into circadian rhythms demonstrated that, when humans are without zeitgebers, or in constant lighting conditions, they have a "free running" period of 24.9 hours. [7] In this experiment, humans were in an underground, sound-proof bunker where the lights were continuously on.