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The Human Circadian System
Simply put, "circadian rhythm" means daily cycle. "Circadian" comes from Latin: circa= approximately, and dies=day.
Many bodily functions and behaviors have a circadian rhythm:
The most obvious circadian rhythm is the sleep/wake cycle. Body temperature and the secretion of various hormones (insulin, cortisol, and melatonin) have circadian rhythms. Cognitive performance and mood also have been found to be synchronized by circadian rhythms.
The circadian rhythm of melatonin:
Melatonin is a hormone made by the pineal gland in the brain and is normally produced at night. Melatonin plays a role in the regulation of the body clock by acting as a darkness signal to the body. When melatonin is produced, the body knows it is time to get ready for sleep.
The endogenous circadian pacemaker:
This is the biological clock that regulates all circadian rhythms. The endogenous circadian pacemaker is located in the suprachiasmatic nucleus (SCN) of the hypothalamus in the brain. The SCN is normally entrained (synchronized) to a 24-hour cycle by time cues in the environment, mainly by the 24-hour light-dark cycle in sighted people. Light in the eyes is the primary synchronizer of the biological clock.
Circadian rhythm disorders:
In circadian rhythm disorders, there is a mismatch between the timing of the sleep/wake rhythm and the desired (or required) time for sleep.
Examples of circadian rhythm disorders include winter depression (Seasonal Affective Disorder, or SAD), jetlag, shift work maladaption, advanced sleep phase syndrome, delayed sleep phase syndrome, and 24-hour sleep/wake schedule disorders (like those associated with free-running circadian rhythms).
Free-running circadian rhythms:
About 15% of the legally blind have no light perception at all. The majority of totally blind individuals have abnormal, free-running circadian rhythms, meaning that their internal body clock drifts to a different time each day. Although free-running blind individuals may try to maintain normal sleep and wake time, they typically have recurrent bouts of nighttime insomnia and daytime sleepiness as their body clocks drift out of sync with the 24-hour day. This problem is one of the primary complaints in some blind individuals. The recurrences of such symptoms can be precisely predicted through circadian rhythm (specifically, melatonin rhythm) assessments.