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AIKEN-AUGUSTAʟS MOST SALUBRIOUS NEWSPAPER ⢠FOUNDED IN 2006
APRIL 13, 2018
AUGUSTARX.COM
The invisible clock Every previous edition of Body Parts, our occasional series featuring components of the human machine, has featured something fairly major: the heart, DNA, hair and others, including the most recent installment in February, tears.
This time the body part is essentially intangible: the internal circadian clock that regulates huge segments of our lives, sometimes in ways that weâre very much aware of, others that subtly occur on a molecular level. Like most clocks, our on-
BODY PARTS, THE OCCASIONAL SERIES
board timekeeper operates on a 24-hour cycle, creating patterns called circadian rhythms. That word circadian comes from two Latin root words: circa, meaning âaboutâ or âapproximately,â and diem, meaning âday.â By definition, then, anything circadian occurs about once a day, or on a daily rotation. The phrase was coined by University of Minnesota professor Franz Halberg in the 1950s, but the phenomenon had been noted as early as the 4th century B.C. Circadian rhythms are not limited to humans. They have been observed in insects, plants, animals, and even fungi and singlecelled bacteria. In one early experiment (1729), a French scientist noticed a pattern of daily leaf movements in a plant of the Mimosa family. The movements continued even when he kept the plant in complete darkness, establishing the difference between reacting to external stimuli versus marching to the beat of an internal clock. Where is our internal clock located? Itâs ticking in a pair of cell groups called the suprachiasmatic nucleus (SCN) located in the brainâs hypothalamus which, if you
put a dot on your nose right between your eyes, would be straight back, more or less in the middle of the head. Specialized photosensitive ganglion cells in the retina communicate directly with the SCN, helping to calibrate the circadian clock, synchronizing it with Earthâs rotation. Indeed, itâs no accident that our circadian clock is directly connected to the daily turning of our planet on its axis. In fact, one of the definitions of a circadian rhythm (youâll recall the fi rst is that it must occur on a daily basis) is that it can be reset when necessary based on information fed from the retina to the SCN. For example, at 3 oâclock on a Friday afternoon in Augusta, itâs 4 oâclock on Saturday morning in Tokyo. If someone flew from Augusta to Tokyo, he would initially experience that circadian disorientation often called jet lag. This happens on a smaller scale twice every year when we go on and off daylight savings time. Whether the change involves a single hour or the 13-hour difference between here and Japan, the SCN will immediately begin the recalibration process. One of the resets would be
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adjusting the daily delivery time for melatonin, a hormone that promotes sleep. Around sunset, the SCN activates the brainâs pineal gland to start producing melatonin, a standing order for most of the night; melatonin is barely detectable in the body during daytime. The body will further adjust its production of melatonin depending upon the season, starting earlier in the evening during winter, later during the long days of summer. Despite mankindâs many advancements in taming the natural world, it seems that in the end, nature often wins. In the context of this discussion, we usually canât fight against circadian rhythms and win. Airline pilots, for example, can cross several time zones in a dayâs work, prolonging their hours spent in daylight or darkness and disrupting normal sleep patterns. Pilot fatigue as a direct result of this constant clock resetting has been blamed as the primary cause or major contributing factor in countless plane crashes. Shift workers at least have the advantage of a steady schedule (usually), but even so, they Please see CIRCADIAN page 3
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