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 that recur approximately every 24 hours in synchronization with the cycle of sunlight and darkness caused by Earth’s rotation. Studies have demonstrated that in some animals, the SCNs controldaily fluctuations in blood pressure, body 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.
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 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 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.
文章第一段讲了一个科学家长期坚持的观点，即SCNs control circadian rhythms, 而circadian respond to light. 第二段通过转折逻辑词however说明开始讲一个新的观点了，但这个新观点只是部分否定了之前的观点，认为除了上述观点讲的SCNs, 还有4中genes控制circadian rhythms. 最后得出结论：circadian clocks in other organs and tissues may respond to external cues other than light.