Line | Scientists long believed that two nerve clusters in |
the human hypothalamus, called suprachiasmatic | |
nuclei (SCNs), were what controlled our circadian | |
rhythms. Those rhythms are the biological cycles | |
(5) | that recur approximately every 24 hours in |
synchronization with the cycle of sunlight and | |
darkness caused by Earths rotation. Studies have | |
demonstrated that in some animals, the SCNs | |
control daily fluctuations in blood pressure, body | |
(10) | temperature, activity level, and alertness, as well as |
the nighttime release of the sleep-promoting agent | |
melatonin. Furthermore, cells in the human retina | |
dedicated to transmitting information about light | |
levels to the SCNs have recently been discovered. | |
(15) | Four critical genes governing circadian cycles |
have been found to be active in every tissue, | |
however, not just the SCNs, of flies, mice, and | |
humans. In addition, when laboratory rats that | |
usually ate at will were fed only once a day, peak | |
(20) | activity of a clock gene in their livers shifted by |
12 hours, whereas the same clock gene in the | |
SCNs remained synchronized with light cycles. While | |
scientists do not dispute the role of the SCNs in | |
controlling core functions such as the regulation of | |
(25) | body temperature and blood pressure, scientists |
now believe that circadian clocks in other organs | |
and tissues may respond to external cues other than | |
light—including temperature changes—that recur | |
regularly every 24 hours. |
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